KOCKS. 73 



as having different meanings, the distinctions are these : 

 Slate is the term used when the rock splits into thin 

 laminae, and when the word is used alone it means clay 

 slate. But there are mica slates, hornblende slates, etc. 

 The word shale is used when the rock, though slaty, is 

 more brittle than slate, and so crumbles readily. Schist 

 includes rocks so coarse that it would not be proper to 

 call them slates or shales. It comes from the Greek word 

 schistos, divided or split. 



Mica slate, or schist (for it is called by both names), has 

 the same ingredients with gneiss, but has less of feldspar 

 and more of mica in it. The scales of mica give a glis- 

 tening appearance to the surface of the slabs, which are 

 used for flag-stones, hearth-stones, and door-steps. Fur- 

 naces are sometimes lined with them. Scythe-stones are 

 made out of varieties that have a fine grain. 



Hornblende slate, or schist, is more durable than mica 

 slate, from the toughness which the hornblende gives it 

 ( 125), and is therefore very valuable for flagging. 



Talcose slate, or schist, is brittle, but is used for fire- 

 stones. 



Clay slate has about the same composition with mica 

 slate, but the ingredients are so finely mixed that they 

 can not be distinguished from each other. The colors 

 are commonly bluish, greenish, gray, or reddish. Slate 

 is used for roofing, and for making drawing-slates, pen- 

 cils, etc. 



150. Quartz Rock. This is composed of quartz, either 

 in the granular or arenaceous (sandy) form. There are 

 varieties which result from the admixture of other sub- 

 stances, as mica, feldspar, etc. In these varieties there is 

 regular stratification, especially in the micaceous, which 

 often cleaves into slabs, like gneiss and mica slate ; but 

 when the rock is unmixed granular quartz there is no ob- 

 vious stratification. Quartz rock is used for flag-stones, 

 hearth-stones, and fire-stones, and in the form of cobble- 

 stones for paving. The fine sand into which this rock 



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