On the Age of the Felstones and Conglomerates of the Pentland Hills. By JOHN HENDERSON, F.G.S.E., read by D. J. BROWN, F.G.S.E. This paper was illustrated by two sections, and specimens of rocks and fossils were exhibited.

The felstones of the Pentland Hills, with their contemporaneous conglomerates and sandstones, have hitherto been considered of Old Red Sandstone age, by Murchison, McLaren, Geikie, and others. Having frequently examined the various exposed sections throughout the district, and from the evidence collected, the author endeavoured to prove that some of these felstones, conglomerates, and sandstones are as new as the upper portion of the Lower Carboniferous.

The first section referred to may be seen on the north-west side of the hills at Clubbiedean, where beds of Carboniferous sandstone and shales, containing Sphenopteris affinis and other well-known Carboniferous fossils, are ruptured, tilted and hardened by the intrusion of the felstones; and these intrusive felstones enclose fragments of hardened shales and limestones, yielding encrinites belonging to these beds, showing conclusively that these felstones are of a more recent age than the overlying carboniferous. The other section referred to occurs about four miles further to the south-west, at Bevelau and Habbies How, where these supposed Old Red Sandstones and Conglomerates may be seen resting on the upturned edges of the Silurian rocks. In these Silurian rocks the author detected a number of felstone dykes, one of which is about 30 feet broad, and may be traced up the face of Harehill, a distance of about 300 feet, where it is covered by horizontal beds of sandstone-the supposed Old Red--which it does not penetrate, while in the valley to the south of Harehill some limestone pebbles were found enclosed in the conglomerates, which contain fossils evidently of Carboniferous age, such as Serpula parallela, &c., showing that these sandstones and conglomerates cannot be of Old Red age as hitherto supposed.

Now, when it is considered that the Lower Carboniferous rocks in this district are everywhere broken up by intrusive felstones and greenstones, while the sandstones and conglomerates of Harehill and the Cairnhills remain almost untouched by igneous action, and lying nearly horizontal and undisturbed, the natural conclusion arrived at is, that these supposed Old Red Sandstones were not deposited until after the igneous forces which have disturbed the Lower Carboniferous in this district were nearly exhausted; and the whole evidence clearly shows that these supposed Old Red Sandstones, Conglomerates, and Felstones of this part of the Pentland Hills must at least be as recent as the upper part of the Lower Carboniferous.

On the relative ages of the Granitic, Plutonic, and Volcanic Rocks of the Mourne Mountains and Slieve Croob, Co. Down, Ireland. By Professor EDWARD HULL, M.A., F.R.S., F.G.S., and WILLIAM A. TRAILL, B.A., of the Geological Survey of Ireland. (Communicated with the sanction of the Director-General of the Geological Survey.)

Having referred to the bold and interesting physical features of the district, which in some respects resemble those of Arran, and which had already been objects of investigation by Griffith, Berger †, and Bryce ‡, the authors observed that there were, as in Arran itself, two varieties of granite. These had been shown by the Rev. Professor Haughton § to differ in composition; the granite of Slieve Croob (consisting of quartz, orthoclase and mica) being a soda granite," and that of Mourne (consisting of quartz, orthoclase, albite, and mica) being a potash granite." Dr. Bryce had expressed an opinion that these two granites belong to different epochs ||.

* Geological Map of Ireland, 1839.

66

"On the Geological features of the North-Eastern Counties of Ireland," by J. F. Berger, M.D., Trans. Geol. Soc. Lond. 1st ser. vol. i.

"On the Geological Structure of the Counties of Down and Antrim," by James

Bryce, LL.D., Rep. Brit. Assoc. 1852, p. 42.

§ Quart. Journ. Geol. Soc. Lond. vol. xii. p. 188, and xiv. p. 300.

Supra cit.

The relative and (as far as possible) the actual ages of these granites still remained to be determined, and in the absence of stratified deposits newer than the Lower Silurian in immediate contact with the granites themselves, the authors believed that conclusions might be safely arrived at by considerations connected with the basaltic and felspathic dykes by which the rocks had on several occasions been invaded.

They had arrived at the conclusion that the granite of Mourne was more recent than that of Slieve Croob by a long interval of geological time; the former being of Upper Palæozoic, the latter of, perhaps, Mesozoic age. These general conclusions were supported by the following considerations.

The granite of Mourne at its margin in some places passes into quartziferous porphyry, and sends offshoots of this rock in the form of dykes into the surrounding Silurian strata, as may be very clearly determined by several examples in the vicinity of Newcastle. Hence the authors inferred that the dykes of quartz-porphyry and felstone which traverse the granite of Slieve Croob might be referred to the age of the newer granite of Mourne; and thus the greater antiquity of the Slieve Croob granite might be determined.

Trap-dykes-The trap-rocks of the district were classed mineralogically as follows:-(a) Quartz-porphyries and highly silicated felstones. (b) Diorites. (e) Basalts or Dolerites of two ages.

Considered with reference to relative ages of formation, the following was the order of succession, in the ascending series.

(1) Older Basalts and Dolerite Dykes.-These form by far the most numerous of all the trap-rocks of the district, occurring in great numbers along the coast south of Newcastle, and amongst the interior mountains, as at Slieve Muck; they are also unquestionably the oldest of all the trap-rocks of the district.

Their age, with reference to the granite of Mourne, was placed beyond question by a large number of examples in which these dykes, after traversing the Silurian rocks, are abruptly terminated at the margin of the granite; they are therefore older than the granite itself. These older basalts were found to traverse the Silurian rocks in well-formed dykes within vertical (or nearly vertical) walls, and are generally fine-grained, of dark green colour, undistinguishable from those of newer Tertiary age. Sliced specimens showed under the microscope the composition to be augite, triclinic felspar, and titano-ferrite.

(2) The next in order of age are the quartz-porphyries and felstones, which (as already stated) branch off from the main mass of the Mourne granite, and are unquestionably of the same age as the granite itself, and often strongly resemble it in its more compact form. Dykes of these rocks are also found traversing the older granite of Slieve Croob. They consist of a felspathic base with crystals of felspar, grains and crystals of quartz, and sometimes mica or hornblende, as accessories, and in small quantities.

(3) The Diorite dykes are few in number, the finest example occurring at Rostrevor. It consists of a crystalline granular aggregate of reddish felspar and hornblende well developed, and traverses the older basaltic dykes; but is, they believe, older than the granite of Mourne. It is therefore referable to some intermediate period.

(4) Besides the older basaltic dykes, which are cut off by the granite, there are a few which traverse both the Silurian rocks and the granite of Mourne itself. These are therefore newer than those previously described; and as they appear to be connected with those which are found traversing the Cretaceous rocks in Co. Antrim, the authors consider them to be of Miocene age.

In general aspect there is no decided difference between the older and newer basaltic dykes; they have all the external appearance of the Tertiary dykes, which abound along the margin of the basaltic plateau of Antrim, and in the West of Scotland; and had it not been for their relations with the granite of Mourne, they might have all been included in the same category.

It might have been supposed that microscopical examination would show some

Sir Richard Griffith has informed one of the authors that he was already aware of this fact, but had not published his observations. Some of these dykes are represented on his Geological Map of Ireland.

distinction in the basalts of these geological ages; but recent investigations by Zirkel, D. Forbes, Allport, and others tend to show that there is no criterion of age amongst the constituents of basalt, dolerite, or melaphyre; and the presence of olivine, once supposed to be distinctive of Tertiary basalts, has been detected amongst those even of Carboniferous age

Age of the Older Basalt.-The geological age of these older basalts can only be relatively determined. They are newer than the Lower Carboniferous rocks, which they are seen to traverse at Cranfield, Point and Carlingford. Recollecting the abundant evidences of contemporaneous volcanic action which the Carboniferous rocks of Scotland and portions of central England present, the authors are disposed to refer these older basalts to the Upper Carboniferous period itself; and having regard to the prodigious number of these dykes traversing the rocks at intervals along the coast from Dundrum Bay to Carlingford Bay, they suggest the former existence of one or more volcanic vents in their vicinity during later Carboniferous times; such a volcanic focus as is inferred to have existed in the vicinity of Carlingford by Professor Haughton †.

Sequence of Granite, Plutonic, and Volcanic rocks in the Mourne district.The following may be regarded as the order of succession of these rocks with their approximate ages in the district north of Carlingford Bay, all being more recent than the age of the "Caradoc" or "Bala" beds of the Silurian epoch. Commencing with the oldest, we have:

(a) Metamorphic granite of Slieve Croob, Castlewellan, and Newry. Pre-Carboniferous, therefore Paleozoic.

(b) Older basaltic dykes of Mourne and Carlingford. Upper Carboniferous. (c) Diorite Dykes. Later than the Carboniferous.

1. Granite of Mourne .

(d) 2. Felstone and porphyry dykes penetrating the granite

of Slieve Croob and the older basaltic dykes

Post Carboniferous.

(e) Newer Basalts of Miocene (Tertiary) age. Judging by the comparative scarceness of the newer Tertiary dykes in the district of Mourne, the authors drew the conclusion, that it may be considered as the southern limit of the region affected by the volcanic outburst of the Miocene period, which so powerfully affected the district lying to the north-east of Ireland and extending into the Inner Hebrides; while on the other hand it was the seat of active volcanic energy during an earlier period, which in all probability may be identified with the Upper Carboniferous, or that of the Upper Coal-measures of England.

On the Coal-beds of Panama, in reference mainly to their Economic Importance. By the Rev. Dr. HUME.

On the Silurian Rocks of the Counties of Roxburgh and Selkirk.
By CHARLES LAPWORTH and JAMES WILSON.

The authors gave a short summary of what they had already accomplished on the investigation of these strata, which they held fell naturally into five great divisions in this district. These divisions they had named respectively,

* Mr. S. Allport, Geological Magazine,' vol. vi. p. 115.

+ Quart. Journ. Geol. Soc. vol. xii. p. 193.

Professor Harkness suggests that the eruption of the granite of Mourne may be referred to the period which intervened between the depositon of the Carboniferous and Permian strata, a period of great duration; and he thinks there is a strong resemblance between the granite of Mourne and that of Kirkcudbrightshire, which is referable to this period. Against this view it is to be observed that it would bring the older basaltic dykes close upon the heels of the Mourne granite, which seems rather improbable.

The Hawick and Selkirk rocks fill up all the central portion of the district described, extending from near Selkirk to Mosspaul. They form the great anticline of the South Scottish Silurians, and appear to be the lowest rocks exhibited. They contain a few fossils, such as Annelida, Protichnites, Protovirgularia, Phyllopoda.

The Moffat series is remarkable for the bed (or beds) of anthracitic shale which it contains, and which is famous for the large number of Graptolites found in it. The Moffat series, with its black shale-band, makes its appearance twice in the district described,-1st, in the country between Selkirk and Melrose; 2nd, in the region of the Moorfoot Hills; these beds yield fossils of the genera Dicellograpsus, Dicranograpsus, Cladograpsus, Climacograpsus, Discinocaris, Peltocaris, Siphonotreta, Lingula.

The Gala group lies in the syncline formed by these two appearances of the Moffat series, and consists of grits, sandstones, shales, and conglomerates, that imbed a Middle Silurian fauna, including Monograpsus, Diplograpsus, Retiolites, Dictyonema, Aptychopsis, Ceratiocaris, Dictyocaris, Orthoceras.

The Riccarton beds fill up all the Silurian country to the south of a line drawn from Kirkcudbright to Jedburgh. The fossils are Upper Silurian, and include Cyrtograpsus, Ptilograpsus, Theca, Orthoceras, Ceratiocaris, Aptychopsis, Pterygotus, Rhynchonella.

The authors believe that the anthracitic bed of Moffat is of Bala age, that the Gala group contains strata of both Caradoc and Llandovery age, and that the Riccarton beds should be classed with the Wenlock or Lower Ludlow.

On the Graptolites of the Gala Group. By CHARLES LAPWORTH. The Graptolites found in the Gala group form an assemblage quite distinct from that afforded by the Moffat series. The species known at present are :—

Two of these species, i. e. Retiolites obesus and Graptolites socialis, are new to science.

In Retiolites obesus the frond is diprionidian, ensiform, or elongate-elliptical in form, with a length of 1 inch in the largest specimens, to a breadth of more than of an inch. The meshes on the central surface are hexagonal, of an inch in diameter. Round the inner margin of the frond runs a series of large subquadrangular meshes, which forms a peculiar and characteristic braiding, distinguishing this form at once from all other species of the same genus. These meshes show the place of the cellules, which are from 22 to 24 to the inch.

Graptolites socialis is monoprionidian, flagelliform, of an inch in width and less than 2 inches in length. The cellules are formed after the type of those of Graptolites Beckii (Barr.). They are arranged along the concave side of the stipe, from 34 to 44 to the inch.

This species is found in great numbers in some of the Gala beds.

On the Origin of Volcanoes. By P. W. STUART MENTEATH, The author's views are briefly stated in 'Scientific Opinion' for April 7, 1869. Since that date, M. Fouqué in France, and Peschel in Germany, had published very similar views, although M. Fouqué, until lately, opposed all chemical theories of the origin of volcanoes. The author, therefore, ventured to bring forward his theory more in detail, and he believed that if chemical geology were more gene

rally studied, that theory would not appear startling. He had considered the objections of Bischof and others to chemical theories, and he believed that they did not hit the explanation he proposed. That explanation attributes the force of volcanic action to solar energy, stored up in rocks by buried organic matter-this organic matter either existing in rocks as carbon and carbonaceous compounds, or represented by sulphides and other substances, produced by the reducing-action of organic matter. Volcanoes, as has been said of steam-engines, are worked by "the light of other days." Starting from the five groups of well-preserved extinct volcanoes in Spain and Portugal, proceeding to consider the volcanoes of the Mediterranean basin, and finally volcanoes in general, the author concluded that, as had been pointed out by Sterry Hunt, volcanoes, as a rule, lie on or at the borders of much sedimentary rock; and the exceptions to this rule he considered to be explicable in conformity with his theory. These sedimentary rocks, especially in the Mediterranean basin and under the volcanoes of Catalonia, could be said to contain much organic matter. Next, he examined the alleged fact of the occurrence of volcanoes along great lines of fissure, and concluded that their occurrence in lines was due to their connexion with the sea, as well as with lines of sedimentary deposition. The author believes that the sometimes alleged identity of volcanic rocks was a statement either misleading or meaningless, and that the composition of volcanic rocks was just what we should expect, if they were formed from masses of sedimentary rocks, in presence of seawater. Proceeding to the consideration of the results of Fouqué, Deville, Daubeny, and others, regarding the gaseous products of volcanoes, he showed that these afforded striking evidence that a mixture of gases, similar to that evolved in gasworks, was oxidated in volcanoes with production of great heat. To this heat, and to the burning of separated carbon, sulphur, and probably iron, he attributed the high temperature present in some lava on its appearance in the air. From the researches of Sorby, Zirkel, Daubrée, Delesse, Stoppani, and others on the subject of lavas, he concluded that these were formed at moderate temperatures, and only exceptionally fused by the great heat produced in the crater. The enormous amount of heat assumed to be present in volcanic action was, in the author's opinion, in great part mythical, and what was actually ascertained could be explained by the nature of the substances oxidating in the earth and burning at the crater. As to the introduction of air and water, he referred to the penetration of sea-water at Cephalonia, to the researches of Delesse, to the Catalan trompe, and to the fact that sea-water dissolves much oxygen; while the nitrogen evolved, in volcanic areas and elsewhere, is usually either pure or accompanied by less oxygen than would compose atmospheric air. He then pointed out that the amount of carbon found in rocks might be adequate to produce all the heat required, if we assumed the rocks to have been rapidly deposited; whereas, if they had been slowly deposited, the amount of carbon now existing in them could only be a remaining fraction of that they formerly contained, the rest having been evolved as carbonic acid. If he were to reject geological time, as some have done, he might assume that the volcanic heat to be accounted for was just as much as the average amount of carbon was adequate to supply. After attributing the origin of the vast amount of buried carbon now in rocks to buried carbon in former rocks, and remarking that it must have passed very gradually through the atmosphere, he discussed some correlated processes in nature which would keep volcanic action roughly uniform, the sun-force continually passing through organic matter into volcanic heat. He confined himself chiefly to volcanic action proper, as that was generally considered the best evidence of the original-heat theory; but he considered that such general internal heat as had been ascertained might be attributed to the distribution of volcanic heat by water, to general oxidation of the carbon almost universal in rocks, to friction as shown by Bianconi, and finally, to the electric currents ascertained to exist in the earth, and to be probably produced in great part by the sun.

The paper was illustrated by sketches taken by the writer in the Two Sicilies, the Greek Isles, Catalonia, &c., also by some curious specimens of metamorphosed glass, which he had found while excavating for antiquities in Ischia.