GEOLOGY. 



GEOLOGY. 





crust should happen to relieve the extension of the solid or fluid 

 nucleus. During the earlier periods of refrigeration such critical 

 disruptions may have been frequent ; at later times they would occur 

 after long intervals, with greater violence; and finally, when solidi- 

 fication had gone to a certain depth, there might be no subsequent 

 paroxysm, so long as no external agency came to aid the interior 

 tension. 



It may perhaps be worth remarking that the atmosphere of the 

 earlier eras of the world known to geology must be supposed to have 

 transmitted light, much as happens at present, else we should not 

 find the eyes of fossil trilobites constructed as are those of analogous 

 Crustacea at this day. (Buckland, ' Bridgewater Treatise.') 



Secondary Periodt of Geology. On the undulated bed of the sea, 

 round the ranges of primary rocks raised in insulated tracts by great 

 convulsion, secondary strata were formed, sometimes evidently derived 

 from the waste of the primary strata, through the influence of atmos- 

 pheric agency, or the wearing of the sea on its shores. But a consider- 

 able portion of these strata is of purely marine origin ; the calcareous 

 strata may be considered as derived from chemical decomposition of 

 the sea- water, separated from it by the vital functions of Mollusca and 

 Zoophyta, or generated by springs rising in the sea and loaded with 

 carbonate of lime. Of all these modes of formation modern nature 

 offers illustrations, some of them so extensive as to admit of com- 

 parison with many of the ancient limestone rocks. Others of the 

 secondary rocks appear to have been formed of ejected volcanic matter, 

 ashes and scoriae, which by diffusion in water have settled into deposits 

 of considerable extent. The total thickness of the secondary rocks 

 ia but small when compared with that of the primary groups, nor are 

 they, it is probable, spread over such extensive areas; but in the 

 variety and number of alternations of the different sorts of rocks 

 and in the diversity of their imbedded organic fossils they are alto- 

 gether superior. There is among them far more of the differences 

 which separate oceanic from littoral deposits ; and we see abundant 

 proof that during their aggregation the arrangements of nature were 

 extremely analogous in general features to what we now see, however 

 great and numerous the points of difference may be. 



On an attentive consideration of the several systems into which the 

 secondary strata are grouped, namely (in the order of superposition), 

 Cretaceous System ; 

 Oolitic System ; 



Saliferous, or New Red-Sandstone System ; and 

 Carboniferous System, 



it will be perceived that to each of them belong littoral, marine, and 

 oceanic deposits ; sandstones having been chiefly collected amid the 

 agitation of the shores, clays accumulated in quiet bays or gulfs, and 

 limestones aggregated in deeper water ; and in each system, each of 

 these classes of deposited rocks contains somewhat characteristic, if 

 not entirely peculiar : sandstones, more or less felspathic, dark bitu- 

 minous shales, and gray limestones occur in the carboniferous system 

 red or blue colours belong to the sandstones and clays, and magne- 

 eiau combinations to the limestones of the next incumbent rocks; 

 light-coloured sands and pale-blue clays, with yellowish limestones, 

 mark the oolitic system ; and green or ferruginous sands, marly clays, 

 and soft white limestones distinguish the cretaceous rocks. 



These distinctions are important, as guiding us to a right general 

 view of the changes of physical conditions which occasioned them. 

 These it is probable related chiefly to hydrography, and when we have 

 geological maps complete enough to make the required comparisons 

 as to the extent and distribution of the rocks, it appears possible that 

 the direction of oceanic currents, the lines of ancient boundary of 

 land and sea, may become sufficiently known to determine the par- 

 ticular subterranean movements which introduced new conditions and 

 produced new deposits in a given basin of the secondary ocean. 



Each of the great systems alluded to is characterised by the plants 

 and animal remains which lie in it : the Lepidodendra of the carboni- 

 ferous sandstones and shales yield place to the Voltzia of the red- 

 sandstone, and the Cycadacecc of the oolites ; the Products of the 

 carboniferous limestone are never seen among the oolites, which 

 abound with Trigmiae, Pholadomya:, &c., nor in the chalk, from which 

 these forms are absent ; ammonites belong to all the systems, but the 

 groups differ in each; belemnites are confined to the two upper; 

 hamites, scaphites, &c., are scarcely met with out of the cretaceous 

 rocks. 



These statements might be enormously multiplied [OncANic 

 REMAINS], but enough is said to show that the great features of 

 lithological distinction are accompanied by striking characters of 

 organic remains. These characters, so far as marine life is concerned, 

 may evidently be understood by the same inference of a change of 

 oceanic currents ; but the differences of the vegetable world seem to 

 bespeak a general change of the characters of climate. 



Reviewing the four systems in succession, we shall find circum- 

 stances in each strongly indicative of peculiar combinations of the 

 physical agencies of nature. 



Carboniferous System. To what shall we ascribe the abundance of 

 vegetation which furnished the materials of our coal strata? an 

 abundance so great as, upon any hypothesis of accumulation on the 

 spot where the plants died, or in the sea to which currents drifted 

 them, appears to have no parallel, unless amongst the most umbrageous 



forests of Tropical America. By the gradual decay and periodical 

 transport of the woods on the Mississippi or Orinoco we may perhaps 

 best understand the accumulation of many beds of coal, alternating 

 with a far greater number of much thicker earthy sediments ; but 

 even these aboriginal forests seem unequal to produce such enormous 

 coal deposits as we find in Britain and other parts of the northern 

 zones of the globe. The circumstances, whatever they were, which 

 favoured this development of vegetable power, were never repeated, 

 at least in these zones, though deposits of a similar nature to the 

 series of coal strata, and likewise containing fossil plants and thiu 

 beds of coal, diversify the sand and sandstones of the oolitic, creta- 

 ceous, and tertiary eras. 



As a general inference we may observe that all the great thickness 

 (2000 or more yards) of the carboniferous system (excepting perhaps 

 part of the old red-sandstone series) is clearly derived from wasted 

 lands or sea-coasts, or from a decomposition of the sea-water by vital 

 or chemical agency. [COAL ; COAL FORMATION.] 



Whatever was the length of time which elapsed during the accu- 

 mulation of the carboniferous strata, it appears to have passed with 

 little disturbance of the level of land and sea ; for not a single 

 example (we believe) is mentioned of any real unconformity of strati- 

 fication in the whole series, from the base of the old red-sandstone to 

 the uppermost line of the coal strata. The ordinary agencies of the 

 atmosphere and the waves were in full employ, and some traces of 

 volcanic eruptions appear in the trap of Derbyshire and the north of 

 England ; but there is not in the accumulation of the often repeated 

 alternations of limestone, sandstone, shale, &c., of the carboniferous 

 system, anything to require the supposition of greater general convul- 

 sions. It was a period not of repose, but of regular and orderly 

 action among the agencies of nature, so far as the parts where now 

 Europe and North America are situated : and the mineral deposits 

 and organic remains are to be compared with existing operations of 

 nature, in order to learn the physical condition of the ancient land 

 and sea. 



After the formation of the carboniferous strata was ended in Europe 

 and America, the long tranquillity of the ocean in these parts was 

 broken by extensive and violent concussion, so that hardly a single 

 square mile of country can anywhere be found which is not full of 

 fractured and contorted strata, in consequence of subterranean move- 

 ments which mostly preceded the accumulation of the next system 

 of strata. 



The relations of land and sea were so greatly changed by these 

 transient convulsions, that the new ridges of land and islands appear 

 to have been variously scattered in the ocean which flowed round the 

 already uplifted Grampian, Scandinavian, and Welsh mountains. An 

 equal or greater extent of laud appears to have been elevated in 

 Ireland, but with less violence and concussion ; and it is remarkable 

 that some of the greatest faults produced at this epoch were almost 

 wholly unaccompanied by the irruption of any igneous rocks, or any 

 other signs of merely volcanic action (Craven fault, great dyke of 

 Tynedale, South Wales coal-field, &c.) 



The Red-Sandstone System, which is deposited upon and around the 

 broken tracts of the carboniferous system, presents us in some respects 

 with new conclusions, which however seem almost equally to apply 

 to the old red formation. No doubt the sands and clays of this 

 system were collected from wasted land and sea-coasts, and deposited 

 in shallow waters. But whence came the red and greenish colours 

 so characteristic of these strata and the analogous old red formation ? 

 The grains of sand which compose much of the rocks are not red, but 

 white rolled quartz sand, surrounded by red peroxide of iron like a 

 varnish. From none of the older rocks could this abundant red 

 pigment be derived so as to stain the whole sea-bed for 1000 feet or 

 yards in depth. It has been thought that volcanic action alone can 

 explain the occurrence of this iron. It is not however improbable 

 that the oceans which deposited the red-sandstone may have held 

 protoxide of iron dissolved in carbonic acid ; and that from this 

 solution the peroxide of iron was precipitated, as is seen in iron- 

 springs at the present day. 



Instead of the great quantity of vegetable matter buried in the coal 

 tracts, we haye in the principal part of the red-sandstones hardly a 

 few insignificant traces so few in England that scattered fragments 

 are valued in geological reasoning ; neither are the marine reliquisc of 

 the magnesian limestones in the midst of the red rocks at all plentiful, 

 except in a few spots. Even taking the richer German series as a 

 type, the red-sandstone rocks must be pronounced singularly deficient 

 in organic fossils ; and as, generally speaking, the same deficiency of 

 organic life belongs to the older red-sandstone below the mountain 

 limestone, it is at least a plausible supposition that the causes of the 

 red colour and paucity of animal life are somehow closely connected. 

 If we imagine that by reason of the great convulsions which followed 

 the carboniferous era new currents were brought into the same areas 

 of the ocean from tracts yielding abundance of new sediments, the 

 extinction of organic life would be the natural consequence, to be 

 followed afterwards by a gradual revival which is nearly the truth. 

 In the magnesian limestones of this system expire many of the forms 

 of the older carboniferous period, and at higher levels (as in Hie 

 Muschelkalk) we find a strong resemblance of the marine Zoophyta 

 shells and Crustacea to those of the younger oolitic system. Upon 



