86 SCIENTIFIC AGRICULTURE. 



HORNBLENDE SLATE. 



Hornblende predominates in this rock, over the various 

 quantities of quartz, feldspar and mica which it sometimes 

 contains. ' When it contains much feldspar, it is not slaty, but 

 resembles greenstone. It is of a dark color, commonly asso- 

 ciated with, and passes insensibly into clay slate, mica slate 

 and gneiss. 



CLAY SLATE. 



This rock is composed mostly of fine clay, and is usually 

 more or less dark and shining from the mixture of chlorite and 

 black lead which it contains. " It may [says Lyell,] consist of 

 the ingredients of gneiss, or of an extremely fine mixture of 

 mica and quartz, or talc and quartz." It passes insensibly 

 into mica slate, talcose slate, or hornblende slate : on the other 

 hand it passes into unconsolidated clay. 



Clay slate is the kind used for roofing: it varies in color 

 according to its composition, from greenish or bluish gray to 

 lead color. This rock, as well as the following, is used for 

 whetstones: the best hones are compact feldspar, and are 

 erroneously supposed to be petrified wood. 



MICA SLATE. 



This rock is a compound of quartz and mica, the mica being 

 in the greatest quantity. This is one of the most common 

 and abundant of the stratified rocks. It sometimes contains 

 beautiful twelve-sided crystals of garnet in considerable abun- 

 dance : beds of pure quartz also occur in this rock. 



PRIMARY LIMESTONE. 



This rock is sometimes in thick beds of white, bluish, 

 greenish or gray granular marble, such as is used in sculpture : 

 it sometimes contains, mica, quartz, hornblende, feldspar and 

 talc. It is both stratified and unstratified; sometimes being in 

 thick beds without any marks of mechanical arrangement, and 



