82 



SLATE. 



and dry it in the siin, we find that it will split into 

 similar shapes. In like manner, sandstones and 

 other rocks expand by the application of heat, and 

 contract again on cooling; and it is supposed by 

 geologists that large portions of the earth's crust 

 have, in the course of past ages, been subjected 

 again and again to very different degrees of heat and 

 cold, which alterations of temperature have prob- 

 ably caused the phenomenon in question. As such 

 a structure, however, is most conspicuous among 

 rocks of the finest texture, it is supposed by some 

 that it is owing to the influence of crystallization. 

 In proof of this, Scoresby states that the icebergs 

 of Spitzbergen are full of rents, extending perpen- 

 dicularly downward, dividing them into innumerable 

 columns. On the other side, however, it is stated, 

 that in Saxony, where masses of basalt rest on 

 sandstone, the aqueous rock has, for the distance of 

 several feet from the point of junction, assumed a 

 columnar structure similar to that of the trap ; also, 

 that hearthstones, after exposure to the heat of a 

 furnace, without being melted, have become pris- 

 matic. The most probable conclusion is, that these 

 fissures are the result of different causes, such as 

 some modification of crystalline action, or simple 

 contraction during consolidation or during a change 

 of temperature. Nearly all the metallic ores have 

 been found in slate, particularly lead and copper. 



Carbonaceous matter first begins to appear in the 

 slate rocks, there being none in the primary. Draw- 

 ing-slate has about 11 per cent, of carbon. This 

 rock is also the oldest that contains any organic re« 

 mains. 



2. Transition Limestone. 



Transition limestone differs from the primitive in 

 being less crystallized in its structure, more com- 



Eact and fine grained, more variegated as to colour 

 eing white, red, brown, gray, black, or streaked. It 



