<;I:OLO<;Y 



terised bv its own suite of fossils be had got the 



'.. the historx of along succession of geolo^i 



oaf change- : for the fossils enabled him to recog- 



ni-e each group in whatever part of the country 



it occurred, and ho\\e\ei iilltcll its pet rographical 



might have changed. If three conform- 

 able series of strata occur in the order A, B, C 

 I', superimposed on A, and (' upon It, that order i- 

 never reveised elsewhere. Each term of the series 

 may not always be present either one or more may 

 be ilwent but those that do occur always occupy 

 the same relative position. In such a conformable 

 sequence each group may contain fossils peculiar 

 to itself, but a larger or smaller numl>er will usually 

 iiml to range from one group to another, 

 in even from top to bottom of the whole. The 

 fossil* will, in short, indicate a gradual change of 

 tauiia and llora, as we pass from below upwards 

 old forms disappearing, new forms appearing. But 

 should the middle term of the series- (group B) 

 be wanting, then the passage from A to C, owing 

 to the absence of the connecting forms belonging 

 i.i 15, will be more or less abrupt. A conformable 

 sequence, like A, B, C, points to the persistence 

 of similar physical conditions during a longer or 

 shorter period. If the fossils- in each group indicate 

 a sea of moderate depth while the stratum attains 

 a thickness of several thousand feet, the inference 

 will l>e that sedimentation has taken place during 

 a slow movement of subsidence. In other words, 

 the silting-up of the sea has been retarded by the 

 gradual sinking down of its bottom. On the sup- 

 position that the accumulation of the strata has 

 been a very protracted process, the marine fauna 

 will have undergone more or less modification. 

 Sucli change in the life-forms, however, will prob- 

 ably have lieen very gradual ; soine species remain- 

 ing longer unmodified than others, while a few 

 may persist unchanged through the whole period 

 of sedimentation. In the case of an unconformable 

 sequence where C rests directly on A, the physi- 

 cal conditions have evidently not remained con- 

 nt. After the deposition of A, a movement 

 upheaval has ensued ; the sea has disappeared 

 and land has taken its place. Should laud- 

 conditions have continued for a very prolonged 

 period before subsidence supervened and the area 

 once more became submerged, the marine fauna 

 will, in the meantime, have undergone more or less 

 modification in those regions to which it migrated 

 while elevation was in progress. Thus the sedi- 

 ment (group C) which subsequently accumulated 

 over the drowned land surface would come to con- 

 tain a suite of organic remains that might differ 

 greatly from those occurring in the immediately 

 subjacent group A. And the longer the interval 

 bet ween A and C, the more strongly marked would 

 be the break in the succession of life-forms. Such 

 4 breaks in the succession' are of common occur- 

 rence local and more widely-spread movements of 

 depression and elevation having characterised the 

 formation of the fossiliferous strata everywhere. 

 N\ hen it is rememl>ered that every lied of aqueous 

 ro.-k has been formed out of the ruins of pre-exist- 

 ing rocks, igneous or derivative, or lioth, it is obvi- 

 "ii- that the fossiliferous strata cannot possibly 

 eontain a perfect record of all the forms of life 

 which may originally have been entombed in sedi- 

 ment. Many fossils must have disappeared along 

 with the rocks which contained them. Thus, in 

 the case of such a 'break in succession' as that 

 just described, it is obvious that the strata of group 

 A would be more or less denuded before group C 

 u to be accumulated C would rest uncon- 

 formahly upon A. Nor can we believe that the 

 lite forms of earlier ages were ever more fully 

 represented by fossils than existing faunas and 

 floras wiil be by the remains of living things which 



are now being buried in sediment. Of the myriad* 

 of existing i'-ii<--tiial plant- and animal- how 

 tVu will leave any relic behind them ! Aquatic, 

 and more especially marine forms, will doubt ]<*) be 

 preserved in far greater variety and abundance ; 

 nut amongst these are many delicately-fashioned 

 and soft bodied creatures which can onlv become 

 fossils by accident, a- it were. Kuch considerations 

 as these should lead us to expect that the fow*ilifer- 

 ous strata, even when these have apparently been 

 accumulated in a continuous manner, will contain 

 a most imperfect record of the pant life-history of 

 the -liil-. Hut notwithstanding this imperfection 

 of the geological record there is yet ample evidence 

 in -how that gradual extinction of ohi and evolu- 

 tion of new fauna- and floras has l>een the rule. 

 Life has been persistent from its introduction, but 

 subject to endless modifications. With thin con- 

 tinuity in geological history it is obvious that any 

 sulidivisions of past time that we choose to make 

 must l>e arbitrary, for the germ, as it were, of one 

 so-called period must have 1>egun in the period 

 that preceded. But, just as in human history it w 

 convenient to use such terms as the 'Middle Ages,' 

 the 'Elizabethan Period,' &c., so in geology it is 

 useful and indeed necessary, for purposes of descrip- 

 tion and correlation, to group the records into 

 so many subordinate divisions. ' Unconformities,' 

 4 breaks in succession,' &c. often enable this to le 

 done with more or less ease ; but in the case of the 

 better- preserved portions of the stony record it is 

 often very hard to say where a division-line should 

 be drawn. 



HISTORICAL GEOLOGY. The forms of life that 

 existed during some prolonged period of the past 

 have a certain fades which serves to distinguish 

 them as a group from the living things that nour- 

 ished in preceding and succeeding ages. And the 

 strata which contain such a well-marked assem- 

 blage of fossils are included under the term 

 System. By this term, then, is understood all the 

 deposits, whether terrestrial, fresh- water, or marine, 

 which accumulated over the earth's surface upon 

 land, in lakes, or in the sea, at a time when the 

 world was characterised by the presence of some 

 particular and peculiar fauna and flora. By com- 

 paring and correlating the fossiliferous strata 

 throughout the world geologists have been able 

 to arrange the various systems in chronological 

 order. The following table shows the larger divi- 

 sions and subdivisions in the order in which they 

 would appear if they all occurred in one and 

 the same section. (Each system will be found 

 described under its own title. ) 

 4. QCATERNAKY OB PosT-TER- ( Recent System. 



3. TERTIARY OR CAINOZOIC. 



2. SECONDARY OR MESOZOIC. 



( Pleistocene 

 I Pliocene 

 I Miocene 

 I Olipocene 

 I Eocene 

 Cretaceous 

 ic 



{Uretace 

 Jurassii 

 Triassic 



'Permian 



Carboniferous System. 

 Old Red Sandstone and Devonian 

 1. PRIMARY OR PALEOZOIC. < System. 



Siluriun System. 

 Cambrian u 



Ar<-h:i an u 



PHYSIOGRAPHICAL GEOLOGY. Under this head 

 is discussed the origin of the surface-features of 

 the land mountains, valleys, &c. The study 

 of causes now in action shows that everywhere 

 rocks are undergoing disintegration, the resulting 

 detritus gradually travelling from higher to lower 

 levels until eventually it reaches the sea. This 

 continuous and universal denudation is easily read 

 in the present appearance of the rocks forming the 

 surface of the land. The phenomena of truncated 

 strata, faults, &c. (see DKM UATION ) demonstrate 



