42 



SECONDARY ROCKS. — ERA OF THE CARBO- 

 NIFEROUS FORMATION. 



LAND FORMED. — COMMENCEMENT OF LAND PLANTS 



We now enter upon a new great epoch in the history of 

 our globe. There was now dry land. As a consequence 

 of this fact, there was fresh water, for rain, instead of 

 immediately returning to the sea, as formerly, was now 

 gathered in channels of the earth, and became springs, 

 rivers, and lakes. There was now a theatre for the ex- 

 istence of land plants and animals, and it remains to be 

 inquired if these accordingly were produced. 



The Secondary Rocks, in which our further researches 

 are to be prosecuted, consist of a great and varied series, 

 resting, generally unconformably, against flanks of the 

 upturned primary rocks, sometimes themselves considera- 

 bly inclined, at others, forming extensive basin-like beds, 

 nearly horizontal ; in many places much broken up and 

 shifted by disturbances from below. They have all been 

 formed out of the materials of the older rocks, by virtue of 

 the wearing power of air and water, which is still every 

 day carrying down vast quantities of the elevated matter of 

 the globe into the sea. But the separate strata are each 

 much more distinct in the matter of its composition than 

 might be expected. Some are silicious, or arenaceous 

 (sandstone,) composed mainly of fine grains from the 

 quartz rocks — the most abundant of the primary strata. 

 Others are argillaceous — clays, shales, &c, chiefly deri- 

 ved, probably, from the slate beds of the primary series. 

 Others are calcareous, derived from the early limestone. 

 As a general feature, they are softer and less crystalline 

 than the primary rocks, as if they had endured less of 

 both heat and pressure than the senior formation. There 

 are beds (coal) formed solely of vegetable matter, and 

 some others in which the main ingredient is particles of 

 iron (the iron black band.) The secondary rocks are quite 

 as communicative with regard to their portions of the 

 earth's history as the primitive were. 



The first, or lowest group of the secondary rocks, is 

 called the Carboniferous Formation, from the remarka- 

 ble feature of its numerous interspersed beds of coal It 

 commences with the beds of the mountain limestone, 



