Изображения страниц
PDF
EPUB

Palma, contain radiating particles of epidote which gradually merge into the mass of the orthoclase. This and similar instances can scarcely be explained otherwise than on the supposition that the formation of the epidote preceded that of the orthoclase. Other facts concerning the occurrence of epidote in syenitic rocks would seem to indicate that the formation of the hornblende pre.ceded or took place contemporaneously with that of the epidote. Senft has observed, near Brotterode, staurolite crystals enclosed in transparent plates of mica, and G. Rose describes both staurolite and cyanite columns as occurring in a similar manner. According to Senft, tourmaline, garnet, staurolite and cyanite are very constant companions of potash mica in crystalline rocks, and most frequently occur bedded in it as well developed crystals, and when separated from the surrounding mass of mica, leave in it an accurately bounded, smooth sided and sharp angled impression of their several forms.*

The order of the formation of the minerals of granite has been a matter of frequent discussion, and the impression prevails that the mica preceded the formation of at least the quartz in that rock. Senft thus gives the result of his observations on this matter: "Potash mica shews itself most frequently associated "with amorphous quartz and with orthoclase; with the first "usually so that it lies imbedded in its mass, which would in"dicate a later formation for the quartz; with the orthoclase, on "the contrary, frequently so that it appears to sit upon it, so "that one must regard the mica as the newest mineral. How

ever, there are not wanting examples of the occurrence of mica "sitting upon the quartz, nor of others in which it appears so evenly intermixed with fresh orthoclase that one must ascribe "to them a contemporaneous origin." †

Senft has also the following remark on the mutual relations of oligoclase and hornblende: "Where oligoclase occurs in very "distinct intermixture with crystals of hornblende, it, for the "most part, surrounds them, and, indeed, often completely encloses "them in its mass. This relation plainly indicates that although "both minerals were produced in one and the same original magma, nevertheless, the hornblende was the first born, and the "oligoclase was obliged to produce itself out of that part of the magma remaining after the formation of the hornblende."

[ocr errors]
[ocr errors]

• Felsgemengtheile, p. 707. VOL. VI.

K

Felsgemengtheile, p. 707.

No. 3.

The study of the manner and order of the formation of crystalline minerals in coarse-grained, compound crystalline rocks, has not, on the whole, had that attention which it deserves. On the other hand many of the results obtained in the microscopical examination of fine-grained original rocks have an important bearing upon this subject. Vogelsang* has described with the most painstaking accuracy his observations on the mutual rela-. tions of the minerals of many pitchstones, trachytes and porphyries. Mention must first be made of a very interesting phenomenon which he has detected in the microscopical structure of many trachytic and porphyritic rocks. This is called Fluidalstructure, and seems to have been discovered somewhat earlier and independently by E. Weiss. This term is to be understood to denote such a position of the constituents of a rock relatively to each other, as to allow of the inference being drawn that a movement of the mass either as a whole or in its smallest parts, had taken place while the process of crystallisation or solidification was going on. Eight different illustrations of this phenomenon are given in the beautifully coloured plates accompanying Vogelsang's work. One of these shews a trachytic pitchstone from the Euganean hills magnified 100 times. In a brownish perfectly vitreous matrix there are found yellowish grains of glassy felspar, needles of hornblende and microscopical crystals of magnetite. The whole of the vitreous matrix is, besides, filled with small prismatic crystals which are sharply distinguishable from the dark ground. These, Vogelsang hesitates to declare to be felspars, and in the meantime, for convenience sake, terms them "microlites." These little crystals are quite frequent in many rocks, and it is possible to distinguish light and dark coloured microlites, the former being in all likelihood scapolites or felspars, the latter augites or hornblendes. The figure shews the position of these little crystals in relation to the larger ones above named, and it is easily observed that the former lie with their longest axes parallel to each other except in the neighbourhood of the larger crystals of felspar, hornblende and magnetite, around certain sides of which they crowd more closely than elsewhere. The drawing shews the effect of the

• Beiträge zur Kemetuiss der Feldspath bildung, Haarlem, 1866. Vogelsang-Philosophie der Geologie und Microscopische Ges

teins-studien-Bonn, 1867.

last movement of the mass at the moment of its final solidification. The observer can plainly see that this movement proceeded from right to left, crowded the microlites against the right sides of the larger previously formed crystals, and then carried them past these in the direction of the flow, namely, towards the left. The figure further shews that one large dark coloured crystal of hornblende had been broken into two pieces, and that the smallest of these, after the fracture, had been caused by the motion of the mass to assume a new position against the end of the larger piece. There can be no doubt, says Vogelsang, as to this fact, for each piece possesses a crystalline and a fractured end, and at the latter, in the larger piece, a crystal of magnetite is seen which corresponds exactly to a space visible in the broken end of the smaller piece. The crystal has evidently been broken at this weak place, and the pieces afterwards turned and pressed against each other. Sometimes the felspar crystals in this rock shew a light brown edge round the clear central mass of the crystal. When more strongly magnified, it becomes plain that the brown vitreous matrix has penetrated the crystal in innumerable places by the cleavage planes. In some crystals this only takes place to a certain depth; others are penetrated through and through by the matrix. Fluid al-structure, sometimes closely resembling that just described and sometimes considerably modified, has been observed by Vogelsang in the basalts of Unkel and Obercassel, in the lava of the island of Ischia, in the diabase of Weilburg on the Sahn, in the quartzose trachyte of Campiglia, in the black pitchstone of Zwickau, and in the quartzose porphyry of Wurtzen in Saxony. Another figure gives a representation of a part of the last named rock magnified 200 times. In this example the Fluidal-structure is not indicated by the position of crystals previously developed, but by a varied colouring which corresponds to differences of densities in the vitreous matrix. A similar appearance is frequently visible in window glass when its substance has not been rendered perfectly homogeneous in the manufacture. Through the whole of the matrix of this rock there are scattered very fine black points, but these are found much less frequently in the dark than in the lightcoloured portions of the matrix.

Many of the facts observed by the naked eye, concerning the order of the formation of rock minerals, are confirmed by Vogelsang's researches with the microscope. Especially

decided is the result as regards magnetite, which is invariably observed to be the oldest formed mineral in the more recent eruptive rocks, all the crystalline constituents of which enclose it. The felspars contained in trachytes, basalts, dolerites, and melaphyres, and the augites and hornblendes of the same rocks, all found the magnetite ready formed when their developement began, and enclosed it as their growth progressed. Even leucite and olivine, which are ordinarily free from foreign enclosures, are found to contain magnetite. On the other hand magnetite is seldom enclosed by quartz, but it is to be remembered that rhyolites very seldom carry the former mineral. In the matrices of many basalts, melaphyres and trachytes, which, in an undecomposed condition, present under the microscope a mass of microlites, the magnetite is found inserted between the needles and determining their limits. The andesite of Lowenburg in Siebengebirge shews, under the microscope, many of these phenomena clearly and distinctly.

In considering the observations that have been made on this subject one cannot avoid remarking that magnetite, tourmaline, and other basic accessory minerals, appear to have been the first to separate from the solidifying magma of crystalline rocks. After the very basic minerals the essential constituents seem to have been formed somewhat in the following order: 1st. Mica; 2nd. Hornblende; 3rd. Felspar; 4th. Quartz. It would, therefore, seem possible to recognise the operation of a definite law in the order of the separation of these minerals from their mother magma, namely, that the minerals of original rocks have crystallised out in the order of their basicity. Some facts, in support of the existence of such a law, are observable in connection with the composition of porphyritic rocks. Not unfrequently the felspar crystals found in these, and which we must suppose, in accordance with facts stated above, to have been produced previous to the solidification of their matrices, have a more basic composition than the latter, or, what amounts to the same thing, the composition of the matrices is more siliceous than that of the whole rock including the crystals. Thus, according to Laspeyres, the felsitic porphyry of Mühlberg, near Halle, enclosing colourless sanidine, oligoclase, quartz and a little mica, contains 72.24 p. c. silica, while the dark greyish green matrix contains 74.41 p. c. Again, the porphyrite of Gänse-Schnabel, near Ilfeld, containing triclinic felspar and other crystals has a silica contents

of 64.34 p. c. The homogeneous, nearly infusible matrix of the
same rock contains 67.36 p. c. of silica. The labradorite por-
phyrite of Mühlenthal, near Elbingerode in the Hartz, possesses
a black, very fresh and hard matrix, which encloses undecom-
posed very lustrous crystals of labradorite, and a dark green or
black augitic or hornblendic mineral. The labradorite contains
51.11 p. c. silica, while the whole rock, in spite of the presence
of the, doubtless more basic, black mineral, contains 57.57 p. c.
silica. On the other hand, in many porphyries and rhyolites
distinct quartz crystals are developed, which, of course, must be
more acid than the enclosing matrix. In spite of this exception,
the law above referred to still applies so far as regards the
minerals developed in crystalline rocks or separated out from
their matrices during solidification.

VIII.-SPECIFIC GRAVITY.

It has been already remarked that in general the specific gravity of original rocks decreases with the increase of silica and increases with the decrease in quantity of the same substance; the most acid rocks are specifically the lightest, the most basic rocks are specifically the heaviest. Abich was the first to call attention to this as exhibited among the volcanic rocks, and to shew the conclusions which might be drawn regarding the silica contents of these rocks from their ascertained specific gravities, Although the same relation has been observed to exist among the granitic and porphyritic rocks, and doubtless runs through all the orders, it has not been found that a certain specific gravity invariably corresponds to a certain degree of silicification or that, for instance, because a syenite containing 59.83 p. c. of silica has a specific gravity of 2,730, a trachyte having the same silica contents will have the same specific gravity. On the contrary we find decided differences as to specific gravity in rocks of similar composition, but belonging to different orders of texture. The following table shews the average specific gravity of the various families of granular, porphyritic and trachytic rocks :

[blocks in formation]
« ПредыдущаяПродолжить »