GEOLOGY OF MOUNT MARCY 45 



The theory that the garnet formation is due to siHca from an 

 apHtic invasion hinges upon one specimen. It is rather difficult to 

 offer evidence either for or against this case. The apHte is very 

 badly strained and so shows that it has been subject to some dynamic 

 metamorphism. It includes some small grains of basic plagioclase, 

 but whether these are part of the already granulated anorthosite or 

 have been included prior to granulation is not definitely shown. 

 The writer is of the opinion that the aplite invaded the rock after 

 most of the granulation had taken place, but this opinion is based on 

 very slight evidence and is not a conviction. Since the aplite has 

 been observed in only one specimen it can certainly not be regarded 

 as playing any important role in the majority of cases and may be 

 disregarded. 



There remains the explanation that the garnet rims are due to 

 reaction between feldspars and ferromagnesian minerals. This 

 process has been suggested by J. F. Kemp and accepted by later 

 observers, but so far as the writer knows there has been no very 

 complete discussion of it. Applied to the specimens in hand, it seems 

 to meet all the facts. The garnets lie between a basic plagioclase 

 ground mass and an aggregate of augite and hypersthene. They are 

 composed of the lime, the magnesia, and the ferrous-iron-bearing 

 garnet molecules as shown by analysis. The hypersthene could 

 supply some of the magnesia, ferrous iron and silica. The augite 

 could supply more of the ferrous iron and magnesia, some of the 

 lime and some of the alumina and silica. The ferric iron of the 

 augite could have gone into the formation of some of the magnetite. 

 From the basic plagioclase the anorthite molecules could add more 

 Hme, alumina and silica. The slides show an excessive development 

 of silica, contemporaneous with the garnet and some of the magne- 

 tite. This also accords with the conditions. To take the simplest 

 case; hypersthene (Mg,Fe)Si03 reacting with anorthite Ca AUSisOg, 

 to form a garnet Ca Mg Fe Al2(Si04)3. This would require two 

 hypersthene molecules and one anorthite and could be written : 

 MgO.FeOCSiOs)^ + CaO.ALOgCSiO,), = CaO.MgO.FeO.Al.O, 

 (Si02)3-f-Si02. The presence of quartz may then be regarded as a 

 partial confirmation of the reaction. The greatest difficulty is to 

 account for the albite molecule which must have been associated with 

 the anorthite in the plagioclase. In the thin sections certain small 

 grains and inclusions were determined as doubtful orthoclase or 

 microline. It is quite possible that these are anorthoclase, and 



