SOME GENERAL OBSERVATIONS OX THE WHOLE PALAEOZOIC. 405 



fishes we can almost find the point of separation of the two great 

 branches, Reptile and Fish, of the vertebrate stem ; and in the former 

 the commencing differentiation of the several orders of Reptiles. All 

 the earliest amphibians had persistent notochord (Cope). 



Some General Observations on the Wliole Palccozoic. 



We have defined geology as the history of the evolution of the 

 earth. Evolution* therefore, is the central idea of geology. It is this 

 idea alone which makes geology a distinct science. This is the cohe- 

 sive principle which unites and gives significance to all the scattered 

 facts of geology ; which cements what would otherwise be a mere inco- 

 herent pile of rubbish into a solid and symmetrical edifice. It seems 

 appropriate, therefore, that at the end of the long and eventful Palaeo- 

 zoic era we should glance backward and briefly recapitulate the evi- 

 dences of progressive change (evolution), physical, chemical, and vital. 



Physical Changes. — The Palaeozoic era opened on this continent 

 with a V-shaped mass of land — the Laurentian area — to the north ; also, 

 a land-mass of Laurentian rocks, of unknown shape and extent, on the 

 eastern border, and probably some islands and masses of larger extent 

 in the Basin and Rocky Mountain regions. This condition of things is 

 represented on the map on page 292. Throughout the Palaeozoic era 

 there was an accretion of land to this nucleus by upheaval of con- 

 tiguous sea-bottoms; a development of the continent southward (and 

 perhaps northward) from the northern area, and both eastward and 

 westward from the eastern border area, until at the end of the Palaeo- 

 zoic the eastern half of the continent included certainly all the Lau- 

 rentian, Silurian, Devonian, and Carboniferous areas shown on the map 

 on page 291, and probably also some on the eastern border of the eastern 

 Archaean area, which was subsequently covered by the sea, and is there- 

 fore now concealed by more recent deposits. The loss of Palaeozoic 

 land on the eastern border probably took place during the Appalachian 

 revolution. In the Rocky Mountain region the development was prob- 

 ably less steady. Unconformity of Carboniferous on Silurian strata 

 shows extensive land-areas there during Devonian times. Thus it is 

 seen that the continent was already sketched in the beginning of the 

 Palaeozoic, and the process of development went on during that era, so 

 that at the end the outlines of the continent were already unmistak- 

 able. TVe shall trace the further development hereafter. 



Chemical Changes. — Progressive changes in chemical conditions are 

 no less evident. At first — i. e., before the Archaean era — before the ex- 

 istence of life on the earth — the atmosphere, as shown by Hunt (Essays, 

 p. 40 et seq.), was loaded with carbonic acid, representing all the car- 

 Ion and carbonates in the world ; with sulphuric acid representing all 

 the sulphur and sulphates ; with hydrochloric acid representing all the 



