Archaia. 489 



life. Their occasional veins of graphite or bands of crystaline lime- 

 stone afford but doubtful evidence of their having once been stra- 

 tified or that they were the habitat of animal and vegetable life. 

 To the strata superimposed upon these primitive rocks geology points 

 us for its evidence of the way in which the earth's crust has been 

 formed and as indices of the time that has elapsed during its form- 

 ation. These stratified rocks have been divided into three great 

 periods, — the Paloeozoic or most ancient, the Mezozoic or middle 

 and the Cainozoic or latest. These divisions are again subdivided 

 into numerous minor strata each determined and in some sense sep- 

 erated from the others by its peculiar organic remains. These 

 strata are shown to be either the result of vegetation, as in the coal 

 measures, or of animal life, as in the Silurian and other rocks, or 

 of aqueous deposition. These three forces if we may so speak 

 have been the agencies by which the earth's crust has been mainly 

 formed. Now to any one who can form an idea of the succession 

 of organic life, the remains of which these strata contain, and of 

 the slow action of aqueous depositions it must be obvious that the 

 crust of the earth is of immense antiquity — that from the period 

 of the earliest Silurian seas up to the recent stratum u;>on which 

 man dwells, there is the unquestionable lapse of countless ages. 

 This is one of the certain inductions of geology concerning which 

 there can scarcely be any dispute. 



It is a further fact than in these strata we find evidences of a 

 constant succession of animal and vegetable life. 



First, there is a long period, the Silurian, in which the lower 

 forms of marine animal life vastly predominate and in which but 

 few traces of vegetable life are found and these exclusively ma- 

 rine or Algoid ? 



A second step leads us into a region in which there is added to 

 the invertebrate life of the first a large and magnificent group of 

 Ganoid vertebrate fishes with some forms of the higher land plants 

 recently discovered by Principal Dawson. 



By a third step upwards we reach the great carboniferous 

 or coal measures, in which we find a thickness of about 10,000 

 feet of fluvio-marine strata and for the first time a predominance 

 of land plants, comprising the two lower members of the vege- 

 table kingdom — the cryptogamic and gymospermic plants. Here 

 also we are introduced to the oldest known reptiles, the discovery 

 of which is in a great measure due to Principal Dawson. 



A fourth step brings us among the great Batrachian reptiles 



