300 A TREATISE ON METAMORPHISM. 



occurrence. — Some form of garnet is a very common mineral in a great 

 variety of the schists and gneisses, including those which are derived from 

 sediments and from all forms of igneous rocks, plutonic and volcanic, both 

 lavas and tuffs. Ordinarily the garnet is a subordinate constituent in these 

 rocks, although in some cases it becomes one of the chief constituents. The 

 mineral has its most widespread occurrence in the metamorphosed rocks 

 which have altered under the influence of mechanical action, or with the 

 assistance of igneous injections, or both. Not infrequently where garnet is 

 particularly abundant combined contact and mechanical action have assisted 

 in furnishing the conditions favorable to its formation. In many instances 

 the garnet develops after the mechanical action has ceased, showing that it 

 was not the movements themselves but the other favorable conditions result- 

 ing therefrom which produced the garnets. It appears, therefore, that the 

 conditions favorable for the extensive development of the mineral are heat, 

 moisture, and high pressure. The mineral garnet is the most important of 

 a group of heavy metamorphic minerals which form under the conditions 

 mentioned. Other minerals which form under similar conditions and are 

 frequently associated with garnet are wollastonite, cordierite, vesuvianite, 

 scapolite, chondrodite, staurolite, andalusite, sillimanite, cyanite, tourmaline, 

 zircon, etc. These minerals are all anhydrous, or nearly so, and mostly 

 of a high specific gravity, many of them having a high symmetry. All of 

 them are formed by the union of silica with bases, and are therefore 

 produced by processes of silication. In many instances this simultaneously 

 involves decarbonation, and this change, as already explained, p. 177, 

 absorbs heat and lessens the volume of the compounds. They are 

 therefore minerals which form normally in the zone of anamorphism. 



Garnet thus produced can not in general be said to have been derived 

 from any single mineral. It is usually the result of the rearrangement of 

 material of two or more adjacent minerals. Dana notes " that when garnet 

 is fused, and the material recrystallizes, the resultant minerals are usually 

 pyroxene, melilite, monticellite, scapolite, anorthite, nephelite, etc. 



This doubtless gives an indication as to some of the minerals which 

 are rearranged under the conditions above described for the development of 

 garnet, which are very different from those of dry fusion. Also it is 



a Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. 8. Dana, Wiley & Sons, New 

 York, 6th ed., 1892, p.447. 



