marble slabs it is the union of the two kinds of decay that leads to so rapid an effacement of the monuments.

(3) Curvature and Fracture.-This most remarkable phase of rock-weathering is only to be observed in the slabs of marble which have been firmly inserted into a solid framework of sandstone, and placed in an erect or horizontal position. It consists in the bulging out of the marble accompanied with a series of fractures. This change cannot be explained as mere sagging by gravitation, for it usually appears as a swelling up of the center of the slab, which continues until the large blister-like expansion is ruptured. Nor is it by any means exceptional; it occurs, as a rule, on all the o.der upright marble tablets, and is only found to be wanting in those cases where the marble has evidently not been fitted tightly into its sandstone frame. Wherever there has been little or no room for expansion, protuberance of the marble may be observed. Successive stages may be seen from the first gentle uprise to an unsightly swelling of the whole stone. This change is accompanied by fracture of the marble. The rents in some cases proceed from the margin inward, more particularly from the upper and under edges of the stone, pointing unmistakably to an increase in volume as the cause of fracture. In other cases the rents appear in the central part of the swelling where the tension from curvature has been greatest.

protected from vertical rain by arching masonwork. A small portion of the marble retaining its crust was boiled in Canada balsam, and was then sliced at a right angle to its original polished surface. By this means a section of the crumbled marble was obtained, which could be compared with one of the perfectly fresh stone. From the dark outer amorphous crust, with its carbonaceous and other miscellaneous particles, fine rifts could be seen passing down between the separated calcite granules, which in many cases were quite isolated. The black crust descends into these rifts, and likewise passes along the cleavage planes of the granules. Toward the outer surface of the stone, immediately beneath the crust, the fissures are chiefly filled with a yellowish, structureless substance, which gave a feeble glimmering reaction with polarized light, and enclosed minute amorphous aggregates like portions of the crust. It probably co ists chiefly of sulphate of lime. But the most remarkable feature in the slide was the way in which the calcite granules had been corroded. Seen with reflected light they resembled those surfaces of spar which have been placed in weak hydrochloric acid to lay bare enclosed crystals of zeolites. The solution had taken place partly along the outer surfaces, so as to produce the fine passages or rifts, and partly along the cleavage. Deep cavities, defined by intersecting cleavage planes, appeared to descend into the heart of some of the granures. In no case did I observe any white pel- Some exceedingly interesting examples of hele such as might indicate a redeposit of this singular process of weathering are to be lime from the dissolved carbonate. Except seen in Greyfriars Churchyard. On the south for the veinings of probable sulphate just re-wall, in the enclosure of a well-known county ferred to, the lime, when once dissolved, had apparently been wholly removed in solution. There was further to be observed a certain dirtiness, so to speak, which at the first glance distinguished the section of crumbled marble from the fresh stone. This was due partly to corrosion, but chiefly to the introduction of particles of soot and dust, which could be traced among the interstices and cleavage lamella of the crystalline granules for some distance back from the crust.

It may be inferred, therefore, that the disintegration of the marble is mainly due to the action of carbonic acid in the permeating rain-water, whereby the component crystalline granules of the stone are partially dissolved and their mutual adhesion is destroyed. | This process goes on in all exposures and with every variety in the thickness of the outer crust. It is distinctly traceable in tombstones that have not been erected for more than twenty years. In those which have been standing for a century it is, save in exceptionally sheltered positions, so far advanced that a very slight pressure suffices to crumble the stone into powder. But with this internal disintegration we have to take into consideration the third phase of weathering to which I have alluded, In the upright

family, there is an oblong upright marble slab facing west, and measuring 304 inches in height by 22% inches in breadth and 34 inch in thickness. The last inscription on it bears date 1838, at which time, of course, it was no doubt still smooth and upright. Since then, however, it has escaped from its fastenings on either side, though still held firmly at the top and bottom. It consequently projects from the wall like a well-filled sail. The axis of curvature is, of course, parallel to the upper and lower margins, and the amount of deviation from the original vertical line is fully 21⁄2 inches, so that the hand and arm can be inserted between the curved marble and the perfectly vertical and undisturbed wall to which it was fixed. At the lower end of the slab a minor curvature, to the extent of % of an inch, is observable, coincident with the longer axis of the stone. In both cases the direction of the bencing has been determined by the position of the enclosing solid frame of sandstone which resisted the internal expansion of the marble. Freed from its fastenings at either side, the stone has assumed a simple wave-like curve. But the tension has become so great that a series of rents has appeared along the crest of the fold. One of these has

in the structure of a substance usually believed to be so inelastic as white marble, is the action of frost. White statuary marble is naturally porous. It is rendered still more so by that internal solution which I have described. The marble tombstones in our graveyards are therefore capable of imbibing a relatively large amount of moisture. When

a breadth of one-tenth of an inch at its opening. Not only has the slab been ruptured, but its crust has likewise yielded to the strain, and has broken u into a network of cracks, and some of the isolated portions are beginning to curl up at the edges, exposing the crumbling, decayed marble below. I should add that such has been the expansive force of the marble that the part of the sandstone | this interstitial water is frozen, its expansive Llock in the upper part of the frame, exposed to the direct pressure, has begun to exfoliate, though elsewhere the stone is quite sound.

force, as it passes into the solid state, must increase the isolation of the granules and augment the dimensions of a marble block. I am inclined to believe that this must be the principal cause of the change. Whatever may be the nature of the process, it is evidently one which acts from within the marble itself. Microscopic examination fails to discover any chemical transformation whica would account for the expansion. Dr. Angus Smith has pointed out that in towns the and lose cohesion from a conversion of its lime into the condition of sulphate. I h ve already mentioned that sulphate does exist within the substance of the marble, but that its quantity, so far as I have observed, is too small to be taken into account in this question. Tae expansive power is exerted in such a way as not sensibly to affect the internal structure and composition of the stone. And this, I imagine, is mos: probably the work of frost.

More advanced stages of curvature and fracture may be noticed on many other tombstones in the same burying-place. One of the most conspicuous of these has a peculiar interest from the fact that it occurs on the tablet erected to the memory of one of the most illustrious dead whose dust lies within the precincts of the Greyfriars-the great Joseph Black. He died in 1799. In the cen-mortar of walls may be observed to swell up ter of the sumptuous tomb raised over his grave is inserted a large uprigat slab of white marble, which, facing south, is protected from the weather partly by heavy overhanging masonry and partly by a high stone wall immediately to the west. On this slab a Latin inscription records with pious reverence the genius and achievements of the discoverer of carbonic acid and latent heat; and adds that his friends wished to mark his resting-place by the marble whilst it should last. Less than eighty years, however, have sufficed to render the inscription already partly illegible. The stone, still firmly held all round its margin, has bulged out considerably in the center, and the blister-like expansion has been rent by numerous cracks, which run, on the whole, in the direction of the length of the

stone.

The results of my observations among our burial-grounds show that, save in exceptionally sheltered situations, slabs of marble, exposed to the weather in such a climate and atmosphere as that of Edinburgh, are entirely destroyed in less than a century. Where this destruction takes place by simple comparatively rapid superficial solution and A further stage of decay is exhibited by a reinoval of the stone, the rate of lowering of remarkable tomb on the west wall of the the surface amounts sometimes to about a Greyfriars Churchyard. The marble slab, third of an inch (or roughly nine millimeters) bearing a now almost wholly effaced inscrip- in a century. Where it is effected by internal tion, on which the date 1779 can be seen, is displacement, a curvature of two-and a-half still held tightly within its enclosing frame of inches, with acundant rents, a partial effacesandstone slabs, which are firmly built into ment of the inscription, and a reduction of the wall. But it has swollen out into a ghast- the marble to a pulverulent condition, may be ly protuberance in the center, and is, more- produced in about forty years, and a total over, seamed with rents which strike inward disruption and effacement of the stone within from the margins. In this and in some one hundred. It is evident that white marother examples the marble seems to have un-ble is here utterly unsuited for out-of-door dergone most change on the top of the swell-use, and that its employment for works of ing, partly from the system of fine fissures by which it is broken up, and partly from more direct and effective access of rain. Eventually the cohesion of the stone at that part is destroyed, and the crumbling marble falls out, leaving a hole in the middle of the slab. When this takes place, disintegration proceeds rapidly. Three years ago I sketched a tomb in this stage on the east wall of Cannongate Churchyard. In a recent visit to the place I found that the whole of the marble had since fallen out.

The first cause that naturally suggests itself in explanation of the remarkable change

art which are meant to stand in the open air in such a climate ought to be strenuously resisted. Of course, I am now referring not to the durability of marble generally, but to its behavior in a large town with a moist climate and plenty of coal-smoke.

II. SANDSTONES AND FLAGSTONES.These, being the common building materials of the country, are of most frequent occurrence as monumental stores, and where properly selected are remarkably durable. By far the best varieties are those which consist of a nearly pure fine silicious sand, with little or no iron or lime, and without trace of

bedding or other structure. Some of them integration. It is a laminated flagstone set contain as much as ninety-eight per cent. of on edge, and large portions have scaled off, silica. A good illustration of their power leaving a rough, raw surface where the inof resisting the weather is supplied by scription once ran. In this instance a thickAlexander Henderson s tomb in Greyfriars ness of about one-third of an inch has been Churchyard. He died in 1646, and a few removed in forty years. gears afterwards, the present tombstone, in the form of a solid square block of freestone, was erected over his grave. It was ordered to be defaced in 1662 by command of the Scottish Parliament, but after 1688 it was repaired. Certain bullet marks upon the stone are pointed out as those of the soldiery sent to execute the order. Be this as it may, the original chisel marks on the polished surface of the stone are still perfectly distinct, and the inscribed lettering remains quite sharp. Two hundred years have effected hardly any change upon the stone, save that on the west and north sides, which are those most exposed to wind and rain, the surface is somewhat roughened, and the internal fine parallel jointing begins to show itself.

In the third place, where a sandstone contains concretionary masses of different composition or texture from the main portion of the stone, these are apt to weather at a different rate. Sometimes they resist destruction better than the surrounding sandstone so as to be left as permanent excrescences. More commonly they present less resistance, and are therefore hollowed out into irregular and often exceedingly fantastic shapes. Examples of this kind of weathering abound in our neighborhood. Perhaps the most curious to which a date can be assigned are to be found in the two sandstone pillars which until recently flanked the tomb of Principal Carstares in Greyfriars Churchyard. They were erected some time after the y.ar 1715. Each of them is formed of a single block of stone about eight feet long. Exposure to the air for about 150 years has allowed the original differences of texture or composition to make their influence apparent. Each co!

Three obvious causes of decay in arenaceous rocks may be traced among our monuments. In the first place, the presence of a soluble or easily removable matrix in which the sand-grains are embedded. The most common kinds of matrix are clay, car-umn is hollowed out for almost its entire bonates of lime and iron, and the anhydrous and hydrous peroxides of iron. The presence of the iron reveals itself by its yellow, brown, or red color. So rapid is disintegration from removal of the matrix that the sharply-incised date of a monument erected in Greyfriars Churchyard to an officer who died only in 1863 is no longer legible. At least one-eighth of an inch of surface has here been removed from a portion of the slab in sixteen years, or at the rate of about three-quarters of an inch in a century.

In the second place, where a sandstone is marked by distinct laminae of stratification, it is nearly certain to split up along these planes under the action of the weather, if the surface of the bedding planes is directly exposed. This is well known to builders, who are quite aware of the importance of "laying a stone on its bed." Examples may be observed in our churchyards where sandstones of this character have been used for pilasters and ornamental work, and where the stone, set on its edge, has peeled off in successive layers. In flagstones, which are merely thinly-bedded sandstones, this minute_lamination is often fatal to durability. These 3tones, from the arge size in which slabs of them can be obtained, and from the ease with which they can be worked, form a tempting material for monumental inscriptions. The melancholy result of trusting to their permanence is strikingly shown by a tombstone at the end of the south burying. ground in Greyfriars Churchyard. The date inscribed on it is 1841, and the lettering that remains is as sharp as if cut only recently. The stone weathers very little by surface dis

length on the exposed side into a trough four to six inches deep and six to eight inches broad. As they lean against the wall, beneath the new pillars which have supplanted them, they suggest some rude form of canoe rather than portions of a sephulchral monument.

Where concretions are of a pyritous kind their decomposition gives rise to sulphuric acid, some of which combines with the iron and gives rise to dark stains upon the corroded surface of the stone. Some of the sandstones of the district, full of such impurities, ought never to be employed for architectural purposes. Every block of stone in which they occur should be unhesitatingly condemned Want of attention to this obvious rule has led to the unsightly disfigurement of public buildings.

III. GRANITES.-In Professor Pfaff's experiments, to which I have already referred, he employed plates of syenite and granite, both rough and polished. He found that they had all lost slightly in weight at the end of a year. The annual rate of loss was esti mated by him as equal to 0'0076 min. from the unpolished, and o'0085 from the polished granite. That a polished surface of granite should weather more rapidly than a rough one is perhaps hardly what might have been expected. The same observer remarks that though the polished surface of syenite was still bright at the end of not more than three years, it was less so than at first; and, in particular, that some figures indicating the date, which he had written on it with a diamond, had become entirely defaced. Granite has been employed for too short a time as a

monumental stone in our cemeteries to afford | chiefly from superficial erosion, but I observed any ready means of measuring even approximately its rate of weathering. Traces of decay in s me of its feldspar crystals may be detected, yet in no case that I have seen is the decay of a polished granite surface sensibly apparent after exposure for fifteen or twenty years. That the polish will disappear, and that the surface will gradually roughen as the individual component crystals are more or less easily attacked by the weather, is of cour e sufficiently evident. Even the most durable granite will probably be far surpassed in permanence by the best of our silicious s.ndstones. But as yet the data do not exist for making any satisfactory comparison between them.

Since the preceding pages were written I have had an opportunity of examining the condition of the monumental stones in the graveyards of a number of towns and villages in the northeast of Scotland, where the population is sparse and where comparatively little coal-smoke passes into the atmosphere. The marble tablets last longer there than in Edinburg, but show everywhere indications of decay. They appear to be quite free from the black or gray sulphate-crust They suffer

a few cases of curvature and fracture. As a contrast to the universal decay of the marble tombstones, reference may be made to the remarkable durability of the clay-slate which has been employed for monumental purposes in Aberdeenshire. It is a fine-grained, rather soft rock, containing scattered tubes of pyrites, and capable of being readily dressed into thin smooth slabs. A tombstone of this material, erected in the old burying-ground at Peterhead, sometime between 1785 and 1790, retains its lettering as sharp and smooth as if only recently incised. Yet the stone is soft enough to be easily cut with the knife. The cubes of pyrites have resisted weathering so well that a mere thin film of brown hydrous peroxide conceals the brassy undecomposed sulphide from view. The slate is slightly stained yellow round each cube or kernel of pyrites, but its general smooth surface is not affected. The lapse of nearly a century has produced scarcely any change upon this stone, while neighboring tablets of white marble, 100 to 150 years old, present rough granular surfaces and half-effaced though still legible inscriptions.

1. THE BARON'S STONE OF KILLOCHAN

IV. THE COLLIERS OF CARRICK

V. AMONG THE VOLCANOES OF CENTRAL FRANCE

V. THE OLD GLACIERS OF NORWAY AND SCOTLAND

VII. Rock-WEATHERING MEASURED BY THE DECAY OF TOMBSTONES

BY ARCHIBALD GEIKIE, LL.D., F.R.S.,

Director-General of the Geological Surveys of Great Britain and Ireland.

IN TWO PARTS-PART II