133 



THE GEOLOGICAL SUCCESSION OF FISHES. 



In discussing this subject it will be 

 necessary to say something about the 

 geological history of the earth. Each 

 geological age had its own peculiar 

 fauna, and to write about any part of 

 it means that we must know something 

 about the particular geological age in 

 which the animals under consideration 

 flourished, and something of the 

 earth's previous history. 



The earth is supposed to be a small, 

 condensed portion of the gaseous ma- 

 terial which astronomers tell us at one 

 time pervaded all space. The heat 

 given off when the gas was condens- 

 ing has been largely converted into me- 

 chanical energy which makes the earth 

 revolve once in twenty-four hours and 

 sends it flying through space. As 

 soon as the earth decreased to about 

 its present size and became cool 

 enough for water to be condensed on 

 its surface, it began to write its own 

 history. Its entire surface may have 

 at one time been covered to a uniform 

 depth by water. If such was ever the 

 case it did not remain so long. The in- 

 terior of the earth was very hot and the 

 crust cooled very irregularly and por- 

 tions of it rose above the surface of 

 the water. Since then there have been 

 two antagonizing forces at work. The 

 heat has caused the earth's surface to 

 become irregular and the water has 

 made a strong effort, which has been 

 partially successful, to reduce all irreg- 

 ularities to the same level. We do not 

 know how long these forces nearly bal- 

 anced each other, but sooner or later 

 dry land appeared in many places on 

 the earth's surface. This was for a 

 long period of time washed by heavy 

 rains while the shores for some dis- 

 tance seaward were worn away by ac- 

 tion of the tidal waves. Much of the 

 land area then sank below sea level, 



and became covered with sand, gravel 

 and the like. The portion which re- 

 mained above the level is called the 

 Archaean. Later a general elevation 

 of the land area brought above sea 

 level much of this land and gravel, 

 forming around the Archaean an in- 

 creased land era, which we call Silu- 

 rian. The time when the sand and 

 gravel was deposited forming this land 

 is known as the Silurian age. Follow- 

 ing this came the Devonian age. After 

 this in the following order came the 

 following geological ages : Carbonif- 

 erous, Triassic, Jurassic, Cretaceous, 

 Tertiary, Quaternary and then the 

 present or Recent age, the one in 

 which we now live. Each of these 

 ages is characterized by the peculiar 

 animals which then predominated, and 

 these animals are known only from 

 their remains imj^edded in the rocks as 

 fossils. 



It may not be out of place here to 

 mention that rocks are usually placed 

 in two great classes, those which have 

 been subject to great heat, melted, or 

 partly so, at one time, then cooled and 

 hardened are called metamorphic or 

 igneous rocks. To these belong such 

 rocks as our granites. Those which 

 have not been changed by heat are 

 called sedimentary rocks, such as sand- 

 stones, limestones, etc. In the former 

 class we find no fossils. If fossils ever 

 existed there, the fusing of the rocks 

 has destroyed them. Sedimentary 

 rocks contain many fossils. The 

 Archaean area contains no sedimen- 

 tary rocks, hence no fossils. Between 

 the close of the Archaean and begin- 

 ning of the Silurian is a long interval 

 of which we know nothing. If any 

 rocks were formed during this interval 

 they are in no place exposed to the sur- 

 face of the earth as are portions of all 



