268 HISTORICAL GEOLOGY. 



the Archaeozoic, as we have already seen, the life, if any, 

 was only of the lowest possible kind. Life-forms had 

 not differentiated into distinct, recognizable species. 

 There was not yet what could justly be called a fauna and 

 flora. Then came the lost interval, represented by the 

 unconformity. Of what took place then we know nothing. 

 When the record opens again with the Paleozoic, we have 

 already an abundant and diversified fauna and flora. 

 Even in the lowest Primordial we find all the great de- 

 partments of Invertebrates, and nearly all the classes of 

 these departments, already represented. It certainly 

 looks like a sudden appearance of somewhat highly organ- 

 ized animals, without progenitors. But we must not 

 forget the lost interval. It is probable that during this 

 period of rapid physical changes there were also rapid 

 changes in organic forms. 



It is for these reasons that the Paleozoic is regarded 

 as opening a new era, and, in fact, the most distinct in 

 the history of the earth. We have explained its distinct- 

 ness from the Archaean below, but we shall find hereafter 

 that it is almost equally distinct from the Mesozoic above. 

 It is separated on both sides by unconformity and by 

 changes in life a distinct volume with, as it were, blank 

 boards on either side. 



Rock-System. There is nothing very noteworthy in 

 the character of the rocks of the Paleozoic. Only this 

 may be said : as compared with the Archaean rocks, they 

 are far less universally thick, metamorphic, and crumpled. 

 In mountain-regions, indeed, they are very thick (40,000 

 feet in the Appalachian), very metamorphic, and very 

 much folded ; but in level regions they are often much 

 thinner, entirely unchanged, and level-lying. For exam- 

 ple, in passing from the Appalachian westward, we find 

 the following four kinds of change : 1. In the Appalachian 

 the Paleozoics are 40,000 feet thick ; they thin out west- 

 ward, until at the Mississippi Eiver they are only 4,000 



