GEOLOGY. 



GEOLOGY. 



978 



The stratified argillaceous rocka below, from the ravity of organic remains and other causes, are not so perfectly understood. The 

 following arrangement, based on the labours of Sedgwick, is however correct, with reference to the succession of deposits in the Welsh and 

 Cumbrian districts. The thicknesses are not exactly known. 



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17. Plynlymmon Rocks 

 Bala Limestone 



/ Argillaceous indurated slate, sanely slates. No fossils yet found in it. 

 ( argillaceous rocks, -with Orbicula, Zofiphyla, and other organic remains. 



Calcareous and 

 Calcareous and argillaceous rocks, with Orbicula, Zoopliyta, and other organic remains. 



~ ' 19. Snowdon Rocks 

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Clay Slate 



Chiatolite Slate 

 Hornbiende Slate 



Mica-Schiit System 

 Gm-i* System 



Variously coloured and indurated argillaceous slale. A few fossils have been observed in Wales. 

 Soft dark slate. No fossils known. 



Soft dark slate, with chiastolite. No fossils known. 

 Soft dark slate, with hornblende. No fossils known. 



No organic remains. The beds of mica-schist, composed of mica and quartz, alternate with 

 gneiss, chlorite-schlst, talc-schist, hornblende-schist, clay-slate, quartz-rock, and primary 

 limestone. 



No organic remains. The gneiss beds, composed of mica, quartz, and felspar, alternate 

 locally with mica-schist, quartz-rock, and primary limestone. 



Primary Periods. Gneiss and mica-schist, two of the most abun- 

 dant of the oldest stratified rocks, appear, as to their substance, to be 

 composed of the same parts as granitic rocks, namely, felspar, quartz, 

 and mica, with great variations of proportions, and some admixtures 

 and substitutions of other minerals, constituting alike granite, gneiss, 

 mica-schist, ic. But the ingredients are not in the same condition ; 

 in the granite all are crystallised ; each mineral is independently a 

 crystal, or moulded in the cavities left between crystals ; in gneiss and 

 mica-schist the felspar, quartz, and mica are rolled or fragmented 

 masses. The character of worn surface of the ingredients, combined 

 with the lamination or stratification of the mass, assures us that 

 aqueous agencies have determined the aggregation of gneiss and mica- 

 schist : the character of the lamination, especially the minute flexures 

 which abound in these ancient rocks, suggests somewhat of pecu- 

 liarity in the condition of the water; and the internal crystallisation of 

 the attrited felspar reveals its origin from the disintegration of granite. 



On the other hand it has been contended that the similitude of 

 the mineral composition of gneiss, or mica-schist, to granitic compounds 

 argues a similitude of origin ; and by some writers gneiss, mica schist, 

 Ac., are regarded even as igneous rocks ; by others it is thought that 

 gneiss and mica-schist are intermediate products between sandstone 

 and granite, retaining the lamination and bedding which indicate 

 their original aqueous origin, but assuming a new mineral composition 

 in consequence of the agency of heat. Neither of these views appears 

 satisfactory ; to give a merely igneous origin to gneiss is evidently to 

 leave out half the phenomena ; to suppose the mineral composition of 

 gneiss the effect of heat operating on a common sandstone will never 

 be allowed by those who have studied the rock as it appears in Zet- 

 land, Scotland, or Norway ; for in all these places it is clear that the 

 granular minerals have not derived their external figure from con- 

 cretionary but really from mechanical action, while their exterior 

 structure is truly crystalline. There is however one mineral fre- 

 quently found crystallised in gneiss and mica-schist, namely, garnet ; 

 and the history of this mineral leaves no doubt that the rocks in 

 which it lies have been pervaded by a general high temperature, 

 enough to affect such a fusible substance as garnet, but not enough 

 to melt any one of the regular constituents of granite. Here then 

 appears decisive testimony as to the degree of heat which the gneiss 

 and mica-schist have experienced. By the operation of this pervading 

 heat the particles of calcareous rocks associated with gneiss and mica- 

 schist have undergone a great change : they have been converted to 

 crystallised marble of various colours and qualities. 



The arguments above advanced, conclusive as we deem them on 

 the subject of the origin of gneiss generally, are not intended to 

 apply to cases where, by reason of this rock being buried at great 

 depths below the surface, extraordinary effects of heat may be expe- 

 rienced. There, no doubt, the gneiss such as we see it, clearly 

 revealing the history of its formation, may be wholly melted and 

 re-crystallised, so as to lose entirely all traces of its origin. Some 

 such cases may occur, perhaps even we may admit that evidence for 

 them exists in uplifted granitic regions; and thus some of the 

 monuments of the earth's early history may have been lost : but 

 that this cannot be the general rule almost every mountain-chain 

 bears testimony. 



In these, the most ancient rocks which exhibit to us the combined 

 effects of aqueous and igneous agency, no traces of animal or vegetable 

 life occur, and the conclusion we adopt on the subject is, that few 

 or none of the organised wonders of nature were then in existence, 

 because the physical conditions of the globe within which the 

 existence of animals and plants is limited were not then established. 

 Only one other view of the subject is worthy of notice. According 

 to the hypothesis of the slow reconversion of stratified rocks to 

 granitic compounds, the want of traces of organic forms in the 

 gneiss and mica-schist is ascribed to the destroying agency of 

 heat on the calcareous matter of shells, corals, &c., and the car- 

 bonaceous substance of plants. That heat will affect such calcareous 

 and carbonaceous compounds in the manner assumed is certain. 

 Ftrhapg it might be difficult entirely to reject the hypothesis in the 

 case of the primary limestones, whose alteration to crystallised masses 

 BAT. HIST. DIV. VOL. It. 



may be thought to have wholly destroyed the structure of the shells. 

 Yet as in the limestone of Teesdale, similarly altered by contact with 

 trap rocks, crinoidal stems retain their forms; and as near granite, 

 trap, &c., vegetable remains are recognised, if not in substance, yet 

 at least by their impressions in the shales or grits; and as, finally, 

 among some rocks of the same mineral nature as gneiss and mica- 

 schist, shells and plants of many sorts appear in the Col du Cliar- 

 donnet in Dauphine", the balance of evidence is decidedly against 

 this extreme application of the theory of metamorphism of rocks. 



Upon the whole then the evidence afforded by a careful examina- 

 tion of the oldest strata, in regard to their mineral composition, 

 structure, and absence of organic remains, supports, we will not say 

 establishes, the opinion that these are not only the most ancient strata 

 which man can trace, but the oldest products of watery action on 

 the globe, aud in a great degree anterior to the origin of organic life. 



The general results to which the study of the earliest systems of 

 i strata lead are these : 



1. They are the oldest aqueous deposits visible on the crust of the 

 globe, and rest on masses which have received their present aspect 

 from the action of heat. 



2. They furnish no proof of the contemporaneous or previous 

 existence of dry land. 



3. They are equally destitute of evidence of the contemporaneous 

 or previous existence of plants or animals in the sea. 



4. The rocks of this ancient system are peculiar in their aspect, 

 and though doubtless derived from disintegrated granite, &c., the 

 constituent particles appear to have undergone much less attrition 

 than those which compose rocks of later date. 



5. These rocks are of such great extent as to approach nearer to 

 universal formations than any of later date. 



As a general inference, it appears that the circumstances which 

 accompanied the accumulation of these rocks were greatly different 

 from what we now behold, since nowhere on the sea-shores are any 

 such products found, nor can we suppose anything analogous pro- 

 ducible in the bed of the sea, unless where some peculiar agitation of 

 water may hasten the disintegration of granite. The impression was 

 very strong among early writers of the entire want of accordance 

 between the causes of those early strata and those now in action. 

 De Luc (' Lettre ' iii.) more reservedly says, " We have no reason 

 to expect that the operations of those times can be explained by 

 specific analogies with what we observe in the present state of the 

 earth." 



And as one general hypothesis, we may say with the followers of 

 Leibnitz and Fourier, that the proper internal heat of the earth was 

 then only just so much reduced as to allow of a peculiar watery 

 action upon its cooling crystallised masses, but not enough diminished 

 to allow of the conditions within which the existence of organic 

 beings is restricted on the earth. 



This hypothesis is independent of the consideration already pre- 

 sented as to the original condensation of the globe, and cannot, we 

 believe, be objected to on the ground of anything known concerning 

 the present state of the interior of the globe ; on the contrary, the 

 temperature of the earth augments as we proceed downwards, and 

 this fact, being general, has been shown by Fourier to be inexplicable 

 except as a consequence of a general high temperature now existing 

 in the earth. The planetary spaces round the earth are colder than 

 any part of its surface (Fourier), and continually abstract heat from it : 

 the globe is continually growing colder though at an insensible rate, 

 and must have formerly been hotter, aud then must have lost heat 

 more rapidly. The obvious conclusion from the mathematical theory 

 of the heat of the globe, coupled with observations of the tempera- 

 ture below the surface, leads to the adoption, as an inference from 

 facts, of the view above proposed as an hypothesis to explain other 

 facts. [GNEISS; MICA.] 



Skiddaw, Cambrian, and Silurian Systems. These argillaceous 

 rocks of the primary series of strata bear the same relation to the 

 gneiss and mica-schist as common clays bear to c ..minon sands in 

 modern nature. Some clays are not really more d'stinct from par- 

 ticular sands in their mineral nature than in the conparative fineness 



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