CH.XI THE GEOLOGICAL RECORD 189 



in the geological record which separate the great " eras " and 

 " systems " of the geologist. These phenomena are admir- 

 ably explained in Professor James Geikie's attractive and 

 well -illustrated volume on Earth Sculpture or the Origin 

 of Land Forms, published in 1898. Here I can only 

 attempt to sketch in outline the successive stages of life 

 which are exhibited in the rocks, and point out some of 

 their most striking features with the conclusions to which 

 they lead us. 



During the latter part of the eighteenth century geo- 

 logists were beginning to obtain some detailed knowledge 

 of the earth's crust and its fossils, and arrived at a first rude 

 division into primitive, secondary, and tertiary formations. 

 The first were supposed to represent the epoch before life 

 appeared, and comprised such rocks as granite, basalt, and 

 crystalline schists. Next above these came various strata of 

 sandstones, limestones, and argillaceous rocks, evidently of 

 aqueous origin and often containing abundant fossils of 

 marine, fresh-water, or terrestrial animals and plants. The 

 tertiary were clearly of more recent origin, and contained 

 shells and other remains often closely resembling those of 

 living animals. It was soon found, however, that many of 

 the rocks classed as " primitive " either themselves produced 

 fossils, or were found overlying fossiliferous strata ; and, by a 

 more careful study of these during the early part of the 

 nineteenth century, the three divisions were more precisely 

 limited the first or " Primary," as containing the remains of 

 Mollusca, Crustacea, and some strange fishes and amphibians ; 

 the " Secondary," by the first appearance of reptiles of 

 many strange forms ; and the " Tertiary," by abundance 

 of Mammalia of all the chief types now existing, with 

 others of new and apparently primitive forms, or serving as 

 connecting links with living groups. 



It is a very remarkable fact, not sufficiently dwelt upon 

 in geological treatises, that this first grouping of the whole 

 of the life-forms of the past into three great divisions, at a 

 time when our knowledge of extinct animals and plants was 

 extremely scanty as compared with what it is now, should 

 still be in universal use among the geologists of the world. 

 The exact limits of each of these great divisions have been 



