294 METAMORPHIC ROCKS 



of changes, according to the amount of compression and mashing 

 which the rock has undergone. Greywacke-slate is fine-grained 

 and parts into plates parallel with the stratification planes. 



Slate and Phyllite. — Slate is a fine-grained, dense, and hard 

 rock, which, when metamorphosed by compression, is cleaved. It 

 results from the transformation of clay shales, fine arkose, and 

 sometimes of volcanic tuffs. Crushed fragments of felspar change 

 into interlocking crystals of quartz and felspar, or quartz and mica. 

 The mineral particles, both original and newly developed, have 

 a parallel arrangement of their long axes and cleavage planes, 

 which determines the cleavage of the rock. In colour, slates are 

 usually drab, or dull dark blue, but they may be brick-red, green, 

 or purple. When fine-grained and regularly cleaved, they are ex- 

 tensively quarried for roofing purposes. Great areas of them occur 

 in Vermont, eastern Pennsylvania, Virginia, and Georgia, south of 

 Lake Superior, and on the western flank of the Sierra Nevada. 



Phyllite is slate in a more advanced stage of metamorphosis, in 

 which the mica spangles are more abundant, and visible to the 

 naked eye, giving lustrous surfaces to the cleavage planes. Like 

 micaceous quartzite, phyllite may often be traced into mica schist. 



Marble is a metamorphic limestone, in which the fragments and 

 particles of organic origin have been converted into crystalline 

 calcite. Magnesian limestones yield crystalline dolomites, which 

 are likewise included under marble. In the process of recon- 

 struction, the fossils and even the bedding planes of the original 

 limestone are usually entirely obliterated. The grain of the rock 

 varies much, from the fine, dense, loaf-sugar-like statuary marble 

 to a very coarse texture of large crystals. Pure limestone gives 

 rise to a white marble, but the presence of organic matter is be- 

 trayed by veins of graphite, which may indicate the lines of mash- 

 ing and flow, along which the rock yielded to the compressing 

 force. Iron and organic matters present in the limestone produce 

 a great variety of coloured and variegated marbles, some of which 

 are of extraordinary beauty. The sand and clay present in many 

 limestones will, on metamorphosis, give rise to a variety of silicated 

 minerals. 



