GEOLOGY. 



QEOLOOT. 



of their constituent particles. In consequence of differences of mag- 

 nitude and density, particle* of clays and sands which are derived by 

 watery action from the same Ma-cliff, avalanche, or glacier, are loon 

 separated, carried to unequal distances, and deposited in distant 

 msssss Such, in many cases, is the true origin of the sandstones and 

 shales of the secondary strata, and processes somewhat analogous 

 may perhaps be supposed to have occasioned the remarkable distinct- 

 ness and even reciprocity of occurrence of the gneiss and mica-schist 

 on the one hand, and the slaty rocks on the other. It is seldom that 

 both of these types of primary strata abound in the same geographical 

 region, though then is little doubt that both are derived from a 

 granitic basis. In some cases we may best Conclude that the mate- 

 rials of the slaty rocks were obtained from the wasted gneiss and 

 mica-schist. 



Enormously thick u these argillaceous mosses are, and extensive its 

 is their geographical distribution, they offer in all countries a general 

 character of aspect which easily arrests the attention and impresses 

 the memory. The colour usually approaches to blue, gray, green, or 

 purple ; the texture is usually fine-grained, but portions are included 

 not very different from sandstone or conglomerate (grauwacke, or 

 olasmoaohist of Conybeare) ; the structure is laminated and bedded 

 more or less perfectly, and often in addition complicated with regular 

 symmetrical joints ; there is another entirely distinct set of such 

 divisional planes called ' cleavage,' traversing the planes of deposition. 

 All these circumstances give to the primary argillaceous rocks a deter- 

 minate aspect. The limited limestones which iuterlamiuate the mass 

 are, seldom so crystalline as those in gneiss and mica-schist, and they, 

 as well as the upper and some other parts of the slaty rocks, generally 

 yield organic remains, occasionally in great abundance. These are 

 almost wholly marine (local deposits of land-plants occur), and the 

 niml belong to invertebrate tribes Zoophyta, Conchifera, Crialacea 

 and augment in number and variety as we pass from the lower to 

 the upper parts of this series of rocks. [ORGANIC REMAINS.] 



From a contemplation of the slaty rocks it results : 



1. They not (infrequently rest on the granitic rocks with scarcely 

 any interposition of gneiss or mica-schist (Cornwall, Cumberland, &c.) 



2. The proofs which they offer of the existence of dry laud are 

 chiefly (or wholly) derived from the organic remains of plants, which 

 are not certainly known among the lower groups, but become tolerably 

 plentiful in the upper parts of the systems. 



3. The marine organic remains, shells, corals, Cruttacea, &c., are 

 very scanty in the older systems, and grow more and more numerous 

 and varied towards the upper strata. 



4. The forms and structure of these earliest known fossil races of 

 animals have no extraordinary degree of simplicity, nor are they con- 

 fined to the lowest or least complicated tribes of Inreiiebratti. 



5. The alterations which the rocks have undergone by the action of 

 heat are general, sufficient in most countries to superinduce new 

 structures (slaty cleavage), but not to destroy the traces of organic 

 remains. 



A greater resemblance appears among these fine-grained strata to 

 the deposits from modern waters than is found in the earlier rocks : 

 there is less of peculiarity in their laminar and stratified structure ; 

 they are more varied ; and the alternations of deposits indicate 

 greater variety of natural processes and new conditions, such as the 

 elevation of land, the wasting effects of the atmosphere, and littoral 

 agitation, might occasion. 



We may suppose, in order to account for the origin and gradual 

 augmentation of the traces of organic life, that the flow of heat from 

 within the globe to the surface was retarded by the effect of previous 

 cooling, and by the addition of the older sedimentary rocks above the 

 granite ; and this is in harmony with the fact that generally the lime- 

 stones of this system are less crystallised than those which are of 

 older date. [CAMBRIAN ROCKS ; CUMBRIAN ROCKS ; SILURIAN 

 STSTIM.] 



Passage from the Primary to the Secondary Period of Geological 

 Time. Before the close of the Primary period we find that some 

 limited tracts of land were reared above the waters, so as to nourish 

 the plants which occur in the grauwacke slates of North Devon and 

 the banks of the Rhine (supposing, with the general opinion, that the 

 foasiliferous rocks of Baden, ftc., are of this age). The sea had become 

 entirely fit for the residence of marine Zoophyta, which abounded so 

 as to constitute reels and islands ; Conchifera and Oaiteropoda forming 

 extensive beds ; Trilobita of many kinds, and a few traces of fishes. 

 These however are chiefly in the uppermost of the primary series, 

 and would be ranked as transition deposits by all geologists who use 

 that now neglected and somewhat hypothetical term. Yet it is 

 impossible not to be struck by the gradation of character which con- 

 nects into one long series the granitoid gneiss and the arenaceous 

 Ludlow rocks ; the fine-grained gneiss and mica-schist with the fissile 

 Snowdon slates and argillaceous Wenlock shale. In proportion as the 

 deposit* on a great scale resemble in character of accumulation those 

 of modern times, so the organic remains appear more and more 

 abundant. Some general change of physical condition, such as per- 

 haps only a change of heat will explain, must evidently bo admitted 

 as an hypothesis to connect together this series of phenomena. 



After the de| tuition of the primary strata, the interior forces of 

 beat, no longer operating by a gradual metamorphosis of the pre- 



viously deposited strata, and by a regulated change of the condition 

 of the sea, appear to have been thrown into a state of critical action, 

 and to have operated on the aqueous deposits of ancient date, as at 

 this day the volcanic fires below affect the sedimentary strata accumu- 

 lated from water above. There is hardly a mountain-range of much 

 importance throughout the world where the effects of great convulsive 

 movements affecting the primary strata cannot be seen : frequently it 

 is ascertained to be the case that these movements happened before 

 the production of any of the secondary rocks ; and upon the whole it 

 is evident that the crust of the globe was broken up and dist 

 and the relative geographical distribution of sea and land materially 

 changed by the disturbance. The effects immediately appear : the 

 introduction of a new order of sodimeutary deposits, with new geo- 

 graphical relations ; the extinction of old and the creation of new 

 groups of organic beings; the commencement of a new act (so to 

 speak) in the great history of the earth. 



What relation do the great convulsions here alluded to bear to the 

 movements of a modern earthquake t They are unquestionably due 

 to the same general force, namely, internal heat : a disturbance of the 

 equilibrium of this force is in each case to be admitted the causes 

 and effects are analogous but is the modern earthquake due to a 

 physical agency of equal intensity with that which occasioned the 

 ancient convulsions of the earth's crust ? The uplifting of a mighty 

 range of mountains is a common event, a characteristic occurrence of 

 early geological periods : minute and partial changes of level accom- 

 pany some modern earthquakes. There is no possibility of explaining 

 the former by the latter, except by taking them as differential quan- 

 tities, proportioned to the time elapsed, assuming that they always 

 (or on an average) operated in a certain direction ; and thus summing 

 an almost infinite series of minute changes to make one decided revo- 

 lution. This is, and must necessarily be, the view of the advocates of 

 the invariable constancy of the measure of natural agencies. 



It is enough, in reply to this speculation, to point to the phenomena 

 which require explanation : they are too mighty in extent, and have 

 too much simplicity and even rarity of character to allow of the 

 faintest belief that this hypothesis can be true. On a minuter inspec- 

 tion this conviction is deepened by the wont of any proof of the 

 occurrence of these thousands of small movements, which must have 

 succeeded one another for the production of the given effect. On the 

 contrary, the enormous and simple displacements, 100 to 4000 feet in 

 a vertical line, and ranging 10 or 100 miles in length; the mutual 

 connection of such faults ; the laws of their relative direction, and 

 other phenomena, utterly reject such on imaginary representation of 

 the measure of primeval igneous agency. A much less improbable 

 view, that the whole movement of a great mouutain-chain was accom- 

 plished by gradual elevation or depression, operating through long 

 time in one direction, is apparently difficult to reconcile with the 

 narrow and steep ridges produced, the numerous and powerful flexures 

 of thick rocks, the sudden and great fractures, and other characteristic 

 phenomena. 



We are therefore driven to believe that the igneous effects of earlier 

 date were far more powerfully and generally excited, at particular 

 epochs, than U now observed to be the case. We may be satisfied 

 that the present aspect of the earth U to be viewed as a period of 

 comparative repose ; a period .of ordinary and regular action, and 

 frequent compensation among the agencies of nature ; and may satis- 

 factorily compare it with the whole or some port of the primary 

 period, but not use it as a measure of the violence which accompanied 

 the transition from one early period to another, and thus, amidst great 

 local or general disorder, restored the equilibrium of the interior and 

 exterior agencies of natural changes. This being supposed, the volcanic 

 excitements of modern date being token as the terms of a series of 

 effects of partial and local disturbances, and re-establishments of 

 equilibrium, there may yet remain residuary phenomena not so com- 

 pensated, till some critical combination of events opens a wide access 

 to the interior energies of heat. It is even probable that such do 

 remain. The cavities left by the ejection of lava under the Andes are 

 probably not all compensated by the sinking of the earth in the 

 vicinity, because of the resistance of the coherent crust of rocks above ; 

 yet such resistance is limited, and it is at least conceivable that some 

 part of that mighty range may fall in, as did a great portion of Papau- 

 dayang in Java (1772). 



What is here concluded to be true at this day for volcanic regions 

 taken singly, may easily bo assumed to be probable for large portions 

 of the earth, wheu the igneous energy was capable of more general 

 results, because 'not determined to many local centres of continual or 

 intermitting effect. But we must not leave out of consideration tint 

 gradually diminishing force of heat in the globe, whether this > due 

 to a gradual lowering of its proper temperature, or a gradual stifling 

 of calorific chemical processes. The loss of that heat by mere radia- 

 tion into the cold planetary spaces is a residual phenomenon of 

 infinitesimal value indeed, but of general application to the whole 

 globe, and capable in long time, and independently of local volcanic 

 action, of amounting to a tremendous force. For the heat of the 

 surface of the earth being determined by the heat of the sun and the 

 cold of its planetary path, the exterior crust would contract lees than 

 the interior nucleus, and it would depend on various considerations 

 whether at all, and after what intervals, a violent crushing of the 



