THE FARMER AT HOME 95 



strata, from a few inches to some feet in thickness, alternating with 

 strata of grit-stone and argillite. The beds run in various directions, 

 generally inclined to the horizon, which is called the dip. They are 

 frequently intercepted by columns of other matter ; and the continua- 

 tion of the bed may be higher or lower than the part from which it 

 seems to have been separted. 



Geology has proved that, at one period, there existed an enor- 

 mously abundant land vegetation, the ruins or rubbish of which, 

 carried into seas, and there sunk to the bottom, and afterwards cov- 

 ered over by sand and mud beds, became the substance which we now 

 recognize as coal. This was a natural transaction of vast consequence 

 to us, seeing how much utility we find in coal, both for warming our 

 dwellings and for various manufactures, as well as the production of 

 steam, by which so great a mechanical power is generated. It may 

 naturally excite surprise that the vegetable remains should have so 

 completely changed their apparent character, and become black. But 

 this can be explained by chemistry ; and part of the marvel becomes 

 clear to the simplest understanding when we recall the familiar fact, 

 that damp hay, thrown closely into a heap, gives out heat, and 

 becomes of a dark color. 



When a vegetable mass is excluded from the air. and subjected to 

 a great pressure, a bituminous fermentation is produced, and the 

 result is the mineral coal, which is of various characters, according as 

 the mass has been originally intermingled with sand, clay, or othei 

 earthy impurities. On account of the change effected by mineraliza- 

 tion, it is difficult to detect in coal the traces of a vegetable structure 

 but these can be made clear in all except the highly bituminous cak- 

 ing coal, by cutting or polishing it down into thin transparent slices, 

 when the microscope shows the fibre and cells very plainly. 



From distinct isolated specimens found in the sand-stones amidst 

 the coal beds, we discovered the nature of the plants of this era. 

 They are almost all of a simple cellular structure, and such as exist 

 with us in small forms, (horse tails, club mosses, and frens,) but 

 advanced to an enormous magnitude. The species are all long since 

 extinct. The vegetation generally is such as now grow in clusters of 

 tropical islands ; but it must have been the result of a high tempera- 

 ture, obtained otherwise than that of the tropical regions now is, for 

 the coal strata are found in the temperate, and even the polar regions. 



The conclusion, therefore, to which most geologists have arrived 

 is, that the earth, originally an incandescent or highly, heated mass, 

 was gradually cooled down, until in the Carboniferous period it fos- 

 tered a growth of terrestrial vegetation all over its surface, to which 

 the existing jungles of the tropics are mere barrenness in comparison. 

 The high and uniform temperature, combined with greater proportion 

 of carbonic acid gas in the manufacture, could not only sustain a 

 gigantic and prolific vegetation, but would also create dense vapors, 



