Imágenes de páginas
PDF
EPUB

primitive, as being merely of a relative nature, and leading to no improper associations of ideas respecting the formation of the earth.

Each of the two classes is again divided into stratified and unstratified rocks; distinguished, as the Doctor rather superfluously informs us, by that difference of disposition which is indicated by these terms. And here we have an instance of his great readiness to dogmatize on the narrow ground of his own observations; for not having himself seen serpentine stratified, in the course of his geological travels, he makes bold to question the authority of most other authors who have favoured the world with their investigations on this subject, and to pronounce that the rock just named is always unstratified. At page 78, accordingly, where he gives us a catalogue of the families or genera contained in his arrangement, we find granite and serpentine set down as the unstratified rocks of the primary class. A little additional knowledge, however, derived from an examination of a considerable track of the latter rock in Shetland, induced the Doctor, even whilst his book was in the press, to correct the statement given in the body of the work, and to construct a fresh catalogue, to be inserted at the end, wherein serpentine is made to appear in the list of stratified rocks. It will be admitted, indeed, on all hands, that serpentine is very rarely found stratified; but this unfrequency could furnish no apology to any individual, however active and ingenious as a collector of facts, for persisting in a statement which, at the best, had no higher a warrant than his own negative testimony.

Besides the two great classes of primary and secondary rocks, there is also a list of what the author is pleased to denominate occasional rocks,-a division which appears to us exceedingly absurd. Gypsum, siliceous schist, and conglomerate rocks, here included in that list, are unquestionably as well entitled to a place in the general system, as either granite or gneiss. All rocks are in a certain sense occasional, there being none that can strictly be called universal, or even such as are certain to be found in any given circumstances. It is to the same rage for innovation, too, that we must ascribe the whim which has led the Doctor to place coal in an Appendix, separated entirely from the mineral substances of which it is connected in nature. The rocks of the coal formation are some of the most interesting of the secondary class; on which account, if coal was to be noticed at all, it ought to have been allowed a place in the trap series, with which it is uniformly and closely associated.

It is true, the Author apologizes for the "presence of coal" in his system, as having no claim to the character of a rock; but immediately adds, "it is intimately connected with the strata in which it lies; and as it must also be treated of in any geological history of these substances, it could not have been omitted without inconvenience." Is not this a sufficient reason, why it should have been arranged and described with the strata with which it is so intimately connected?

So much for the principle of classification adopted by Dr. Macculloch. We have already said that we have no great fault to find with it; for where so much is still arbitrary, every man has a right to choose what he thinks least objec tionable as the ground-work of his system; and where convenience is paramount to every other consideration, an author should be allowed the freest scope in his attempts to simplify the more intricate views of his predecessors. Innovation, however, when pursued for its own sake is bad and in science, especially, all changes ought to be avoided but such as lead to certain and obvious improvement.

We come to the succession of the primary rocks, or the order in which they follow one another in their collocation as mountain masses. The first or lowest place in the series is unanimously assigned to granite; which rock, according to the Wernerian arrangement, is succeeded by gneiss: then comes mica-slate, which is followed by clay-slate, transition rocks, old red sand-stone, &c. A good example of this succession is to be found in Scotland, in the mountain-range, extending from Bræmur by the Spittal of Glen Shee, to Blair Gowrie; and several others, we have no doubt, are well known to the learned Author, who, we are tempted to suspect, has been led by his hostility to the doctrines of the German school, to abstain from introducing into his catalogue any section of rocks decidedly Wernerian. In several parts of his volume, indeed, Dr. Macculloch reminds us strongly of Voltaire in his criticisms on Shakspeare; throwing out sarcastic observations in regard to the tenets of the great professor of Freyberg, whilst the most valuable portion of his work is drawn from the treasures of the Saxon mineralogist.

After mentioning that the relative order among the stratified rocks of the primary class is inconstant, the Doctor observes, that "it is not improbable that a distinction may exist in this case between the larger tracts and the more limited collection of strata." It is, however, undoubted that in many instances there is no such distinction, but that even the largest masses or tracts occur in an uncertain order.

Thus, although the great tracts of argillaceous schist are most commonly found on the confines of the series of primary strata, they sometimes also exist below quartz rock, and micaceous schist; of which Scotland furnishes examples. "It must also be remarked, that it is in general only in the smaller tracts, or collections of strata, that the fact of alternation, or the nature of the relative position, can clearly be ascertained. In the larger masses, the connections are often invisible or unassignable; either from their dimensions and the great spaces which they cover, or from the impossibility of ascertaining truly what bed is uppermost, where the inclinations of the strata undergo a reversal, as they are found to do among the primary rocks." To assign an order in such cases, is beyond the reach of our powers.

"The following list contains, in confirmation of the preceding views, a few examples of the different orders of succession which occur among rocks. The examples are all selected from this country, partly for the purpose of increasing the authority of the statements by permitting them to be easily verified, and partly for that of facilitating the access of the student to a set of facts which are at variance with some of the received geological systems. The localities have been added for the former reason: and they might easily have been multiplied had it appeared necessary. Geologists have recently ascertained that similar uncertainties of arrangement exist in other countries, and the student may consult their writings. The examples are not quoted: as throughout this work, it has been deemed expedient to rely as far as possible on those facts respecting which the author imagines he has received that conviction which is founded on observation."

Succession among the Primary Rocks.

[blocks in formation]
[blocks in formation]

The geological reader will be pleased to see such a variety of examples of the succession and alternation of primitive rocks, as proving that the more ancient part of the earth's surface begins to be better known than it was some years ago, when we were taught to believe that the primary strata were not only every where to be found in a primitive country, but also every where occurring in the same order. The disciples of Werner, misled by a premature induction, were disposed to maintain the entireness and regularity of the primitive formation to an extent which subsequent observation has not confirmed. On the contrary, it is now placed altogether beyond dispute that any one of the primitive rocks may be found in contact with granite, and that they may succeed it in any order whatever. Nay, in one of the above examples quoted from the volume now before us, we

find transition sandstone, or as Dr. Macculloch will have it, secondary sandstone, resting upon the fundamental granite. We give the author great credit for his research in this most laborious field of study, and congratulate him upon the important addition which he has made to our knowledge of facts. Still we cannot help repeating our astonishment that in the wide range of his geological travels, he should not have met with one single section of a mountain where the rocks were placed in the following order:

Granite

Gneiss

Micaceous schist

Argillaceous schist
Limestone, &c.

Next after the catalogue and succession of rocks come two or three very wearisome chapters, on the external and internal general characters by which rocks are distinguished. The only particular here which calls for notice, is the attempt made by Dr. M. to distinguish between the structure of a rock, and the texture of a rock-a distinction which we are quite unable either to perceive or comprehend. Under the former term, he arranges those modifications in which the component parts are either more or less dis tinctly separated, or are thus separable under peculiar circumstances. In the modifications arranged under the head of texture (we use the author's words) it must on the contrary be conceived that the mass is continuous, but that it is so constituted as to present analagous appearances to the former; these being generally, however on a smaller scale, and consisting of parts that cannot be separated. The texture, he continues, is thus an indication of an imperfected and minute stracture. As it is scarcely discernible, except on a fracture, the accidents arranged under it pass thus, on the one side into those comprised under the term fracture, as they do, on the other hand, into those included under that of structure. The indefinite boundaries of the forms of nature do not he concludes, easily permit greater accuracy of language and arrangement; and there are cases moreover, in which the term texture is so convenient as to make us unwilling to part with it; independently of the claim which it has acquired from its use among mineralogists.

Of this laboured exposition we shall only say in the words of Dr. M. himself that it respects an imperfected and minute distinction without a difference.

The twelfth chapter is very important, particularly to the young geologist, and displays much research on the part of

4

« AnteriorContinuar »