페이지 이미지

great distance, to find a cause competent to the production of such an effect. The facts were before him, clearly ascertained and defined; it was obviously his duty, therefore, like a true interpreter of nature, to have abstained from interposing the action of a cause, of the existence of which he had no farther evidence than the phenomenon itself, which he thereby wished to explain. There is, in truth, more of geological hypothesis in Dr. Macculloch's mind, and in his book, than he would be willing to admit in his own case, or tolerate in others; and were we disposed to be ill-natured with him, we could bring forward, from almost every article in his two volumes, such proofs of sectarian prejudice, as would confound his claims to the reputation of an unbiassed inquirer., It is impossible, however, to charge him with want of candour. He states facts fairly, whether they stand for him or against him; and it is only by attempts incessantly repeated, to reconcile discordant facts with the dogmas of a favourite hypothesis, that he betrays his character as a partizan. In the following inferences, however, relative to the Isle of Man limestone, there is nothing to blame; and, as we believe them to be not less logi cally than geologically accurate, we make room for their inser


"In reviewing the whole of the preceding facts, which relate to the stratified and unstratified limestone, the following conclusions seem to be among the most interesting. The whole limestone mass, whether stratified or unstratified, consists of one deposit, or of one series, as is proved by its continuity, and by the identity of the organic remains. The absence of the stratified disposition is therefore no proof that any given limestone does not belong to the class of the secondary rocks, a conclusion equally deducible from the history of the calcareous district of Sky. There is consequently no reason, a priori, why a limestone of even much more recent formation than this should not be found unstratified.

"The absence of organic remains proves nothing respecting the comparative antiquity of limestone; and the crystalline texture is also an imperfect criterion of the geological relations of any given mass of that substance; distinctions which are equally to be made from the same example."

"There appears no definable limit of the extent to which a limestone really secondary may exist in an unstratified state; and the various unștratified limestones which have been described in different parts of the world, as of a period prior to the secondary rocks, require therefore to be re-examined; excepting those which alternate with gneiss, micaceous schist, quartz-rock, or argillaceous schist."

The minute details which respect the islands designated by the epithet trap, are followed up with a " General Comparison of those islands; in which the author endeavours to establish the fact, that, though the unstratified rocks from which they take their name predominate on the surface, the same formation of stratified rocks which characterized the Long Island, is common to both these groups. With this view, he exerts much industry in attempting to trace the gneiss and red sandstone series from the one chain to the other; craving, at the same time, con

siderable scope for conjecture as to what may be concealed by the waves. Granting that he has succeeded in his object, and we are not inclined to dispute the point, we can perceive no purpose that will be served by it, except that of gratifying a little party predilection. It suits, in this case, for example, the views of a Huttonian, to confirm the general doctrine, upon which his hypothesis has so much dependence, that the depositions of the stratified rocks were originally extensive and uninterrupted; and, as the sandstone and schist which compose so large a portion of the islands, could not, upon any principle, have been propelled from the plutonic furnace which supplies them ad libitum with trap and granite, it seemed to the Dr. necessary that he should make out, by an induction of facts, that the proper basis of the Hebrides is a mechanical deposition of stratified rocks.

We have a good deal more of this theorizing spirit in his account of the Sandstone Islands; and as in this place, too, it turns upon the subject of mechanical deposition, we shall abridge a few of the Dr's. remarks. The red sandstone, of which the Summer and Croulin Islands are composed, is said to rest upon gneiss, into which, in many places, it is found to graduate. It is, however, on the neighbouring mainland of Ross-shire that the characters of this rock are best defined, and where the author gets the clearest views of its probable origin. The gneiss which here forms the basis upon which the sandstone mountains are raised, averages about 1000 feet above the level of the sea; but the mountains themselves, which are formed both in groups, and detached at great distances from one another, attain to an altitude of 3000 feet above the ocean, or, at an average, 2000 feet above the gneiss. It is stated, too, that the stratification of these mountains is either horizontal or slightly inclined, whilst their declivities on all sides consist of the broken ends of the strata. In no case, we are assured, does the declivity consist of the surface of a stratum, as in mountains of mica-slate and gneiss; where it is not uncommon to find the opposed declivities formed alternately of the surfaces and the edges of the beds. Upon what principle, then, are these phenomena to be explained? Dr. Macculloch himself remarks, that mountains so formed could not have been produced by the elevation of the strata in the way he imagines schistose mountains to have been produced. He is therefore compelled to rest in the opinion, that their present form is entirely owing to the abrasion, or washing away of their sides; an effect proceeding from we know not what combination of causes, but evidently in very close alliance with the debacle of Saussure. We give the conclusion in

his own words, which best express his meaning: "It follows, therefore, that the whole of this country has been once covered with a body of sandstone, equal in thickness, in certain points at least, to the present remaining portions; the variations of the dip marking the undulations of that mass, when in its entire state. The extreme depth of this deposit, as far as it can now be discovered, may be measured by Kea Cloch, of which the altitude has already been given, (as from 3500 to 3700 feet;) since the strata are there nearly horizontal, and extend from the summit to the base, where their greater depth is concealed by the sea."

It is unquestionably a very bold assumption upon which our author attempts to explain the phenomena of the sandstone mountains in Ross and Sutherland; and, by the sweeping use which he has made of it, he creates a very just ground of fear that geology has not yet relinquished her claims upon hypothesis. For our part, we see no difficulty in regarding the mountains in question as the product of crystallization rather than of deposition; and this opinion is mainly confirmed by the very facts which Dr. Macculloch himself brings forward. In the first place, the sandstone under consideration is not only distinguished by its extreme hardness and crystalline structure, but it is found, both in the islands and in the mainland, to pass into gneiss by a gradual transition through quartz and micaceous schist; the quartz on one side of the mass exhibiting a change into red sandstone, and on the other into gneiss. Again, the sandstone is rarely, if ever, found conformable to the gneiss; the angles at which they meet varying from one degree to ninety. The edges of one set of strata are, in some cases, seen abutting on the sides of a different set; whilst, in all cases, the points of attachment between the various members of the aggregated substances, are so extremely arbitrary as to set at defiance every attempt to reconcile them to any notion we can form of mechanical deposition. We are somewhat at a loss, too, to make out the author's meaning, when he says, "Whatever difficulty may be imagined to exist " in explaining this double relation which the sandstone possesses to the gneiss, there is no reason to doubt the identity of "the whole deposit, as the points of connexion and continuity are nevertheless of frequent occurrence." The deposit here spoken of is described as having a depth of nearly 4000 feet, and as consisting of gneiss, slate, quartz, and red sandstone; all passing into one another, and exhibiting the characteristic properties of the rocks usually called primary: on which account, it greatly exceeds our penetration to find out the theoretical views implied in the doctrine that the whole is a deposit, and an identical deposit. The Dr. has gravelled us here completely.



Having, in this incidental way, said all that we meant to say on the Sandstone Islands, we now come to the fourth division of the Hebrides, or the Schistose Isles. These are subdivided into three classes, namely, "the Slate Isles, the Quartz Isles, and "the Chlorite Isles;" the first comprehending Kerrera, Seil, Luing, and Torsa; the second, Scarba, Lunga, Jura, and Isla; and the last embracing the Craignish Isles, the Isles of St. Cormac, Gigha, and Cara.

The classification here employed is obviously founded upon the predominating substances in the several groups; but it is admitted, at the same time, that the different kinds of slate are not so strictly confined to the geographical positions now specified, but that each may be found where the other would be expected. There is, however, nothing very interesting in this part of the work; for the relations of the several schists to the primary rocks are already sufficiently known, and Dr. Macculloch has not been able to throw any new light upon the subject. In his" General Comparison" of these islands, we have indeed another attempt at geological elucidation; but we cannot extol the success with which it is conducted. Proceeding on the principle that the whole was "originally a series of flat beds," he finds it extremely difficult to account for the present condition of the strata in respect of position and alternation. The Huttonian expedient of elevation by means of heat will not answer in all cases, nor will the co-relative effects of subsidence afford a much better explanation of the facts in question. The simplest theory is that which assumes their actual position as the original one; determined by the manner in which they were crystallized, and by their mode of attachment to the fundamental rock. A good deal, no doubt, may be attributed to the action of the sea in wearing away the strata which compose the shores of the several sounds, and even in forming a number of smaller islands out of large ones; still we think that the inclination of the stratified rocks to the horizon, and to one another, must be traced to the cause which gave them their original form and distribution, and not to any subsequent disturbance.

We decline entering upon the Clyde islands. Arran has been already well described by former travellers, particularly by Professor Jameson, to whom we cannot help thinking the present author is more indebted than he feels inclined to acknowledge. It is rather surprising, indeed, that Dr. Macculloch should have detailed his facts and opinions with the same air of originality as if no one had either made observations, or published books before himself on the Western islands. We do not, indeed, accuse him of wilful or concealed plagiarism. Far

from it; nothing being so probable as that, when two good judges have examined minutely the same track of country, they should coincide, to a great extent, in their report of it; still it is usual, we think, when authors thus find themselves on the same ground that had been trodden by others, and arriving at the same results to which they had attained, to make some acknowledgment of the aid which they may have derived from such labours, whether in the way of directing them to what was most worthy of notice, or of confirming them in the opinion which they may have been led to form. We have made a fair comparison between the present work and the only one which is entitled to be named with it, viz. the "Mineralogy of the Scot"tish Isles," and have found that, in some instances, such as Coll and Egg, for example, the descriptions contained in the latter are at once fuller and more satisfactory. Still, as the author of that publication did not extend his survey to more than one-half of the Hebrides, not having so much as set his foot upon any one of the range usually denominated the Long Island, it is only in a few points that his quartos can be regarded as competing with the smart octavos now before us.

Dr. Macculloch's book, therefore, is unquestionably the fullest and most satisfactory that has hitherto been written on the subject to which it is devoted; and, in particular, it is highly valuable as a collection of facts, noted with great industry, and stated with great candour. Mineralogy has made rapid progress during the last twenty years; and the Doctor has availed himself to good purpose of all the knowledge thereby disseminated in Great Britain. In a word, it presents, in many parts, the model of what a mineralogical survey ought to be; and even in those sections of it where the writer indulges too freely in speculation, bewildering himself and his readers in impenetrable mystery, he fails not, nevertheless, to exhibit a fair statement of the facts upon which he reasons.

We have already insinuated, pretty broadly, that the opinions of Dr. Macculloch, on matters strictly geological, are not entitled to much weight. His notions as to the volcanic origin of trap veins, are not supported by any new evidence; on the contrary, he has adduced several important facts, in the course of his survey, which materially invalidate the arguments formerly employed to establish that point. He has given birth to no new doctrine, and has overturned no old heresy; but he has done what is more important than either he has added considerably to the stock of well-ascertained facts, both in respect to the composition of rocks, and their actual distribution.

« 이전계속 »