CONTACT METAMORPHISM 



137 



a quartzite of this nature is shown in Fig. 13. In this case the 

 original rounded granules are readily recognized from the fact 

 that not merely did they 

 contain small cavities and 

 needle-like enclosures, but 

 exteriorly they were cov- 

 ered with a thin pellicle 

 of iron oxide, while the 

 secondary deposit, which 

 now fills all the inter- 

 spaces, is free from en- 

 closures of all kinds and 

 quite pellucid. 



In many quartzites a 

 shearing force has acted 

 a prominent part, where- 

 by the granules have be- 

 come elongated and more 

 or less pulverized along ^'?- IS-Mierosfcructure of quartzite, ^ow- 

 , . . , ^ mg secondary deposit of silica about tne 



tneir margins by tne original quartz grains, 

 friction of rubbing one 



over the other. In such cases mica and other secondary min- 

 erals are often developed, and the rock passes over into a 

 mica schist. 



Still another form of change, or metamorphism, is that 

 known by the name of metasomaiosis, a process of indefinite 

 substitution and replacement. Through the chemical action 

 of percolating solutions certain constituents of a rock may be 

 leached out and replaced by others in indefinite proportions- 

 It is by such processes that have originated a large share of 

 the serpentinous rocks, dolomites, etc. The mineral olivine, 

 an anhydrous ferruginous silicate of magnesia, passes over into 

 serpentine by a simple process of hydration, and a more or less 

 complete change of its combined iron from the ferrous to the 

 ferric state. Provided there be no loss in silica, this change in the 

 olivine, according to T. Sterry Hunt, must be accompanied by an 

 increase of volume amounting to some 33%. Through the hy- 

 dration of eruptive olivine-bearing rocks, or rocks rich in other 

 magnesian silicate minerals, have originated a large proportion 

 of the so-called serpentines and verd-antique marbles. Many 

 serpentines and serpentinous limestones are derived from meta- 



