CEMENTATION 289 



the igneous rock. A siliceous sandstone or conglomerate de- 

 velops no new minerals in the change, or only in insignificant 

 quantity from the impurities present. The bulk of the material 

 simply crystallizes and forms the white rock, quartzite. Clay 

 rocks undergo more radical change and are usually divisible into 

 distinct zones ; the outermost zone is unchanged ; in the inter- 

 mediate one the shale is changed to a dense slate spotted with 

 biotite, magnetite, or other dark minerals. The spotted slate 

 passes gradually into mica schist, a. rock made up of flakes of 

 mica, with some quartz and felspar, arranged in rudely parallel 

 planes. At the contact the rock is converted into hornfels, 1 which 

 is a very dense substance, looking like trap, and filled with numer- 

 ous silicated minerals, such as hornblende, felspar, and many 

 others which were not enumerated in the chapter on the rock- 

 forming minerals. 



Pure limestone is crystallized by the heat into white marble, 

 but as most limestones contain impurities, they develop, when 

 metamorphosed, a large variety of minerals, such as biotite, gar- 

 net, amphiboles, pyroxenes, etc. Beds of bituminous coal are 

 baked into a natural coke, as in Virginia and North Carolina, or 

 changed to anthracite, as in Colorado, and limonite is converted 

 into magnetite. 



In contact metamorphism, the mere molecular rearrangement 

 and chemical recombination of materials already present in the 

 rock are not the only changes which occur. Two other processes, 

 cementation and injection, frequently produce important results. 

 Cementation is the deposition of mineral matters from solution in 

 the interstices between the granules of the rock. Quartz, calcite, 

 iron oxides, felspars, mica, augite, and other minerals may be 

 thus introduced, and sometimes the quantity of new material 

 brought into the rock is very large. Injection is the penetration 

 of a rock by molten substances which may not only fill up all 

 the minute crevices, but even force their way between the con- 

 stituent granules. The distinction between cementation and 



1 Also called homstone, but as this term is used for flint, it is best to retain it in 

 the latter sense only. 



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