672 THE JOURNAL OF GEOLOGY. 



thus formed never have the sharp, angular outline that they have 

 in the case of the feldspar. Irregular tongues of the aggregate 

 eat their way into the quartz, gradually spreading, cutting across, 

 and separating grains originally continuous, and finally entirely 

 replacing them. Such complete replacement of the quartz is, 

 however, exceptional, and seldom extends through any consider- 

 able mass of the rock. Although the quartz usually lags behind 

 the feldspar in the process of alteration, the degree of change in 

 the two minerals has no constant relation. For, on the one 

 hand, a section whose feldspar is completely replaced by the 

 aggregate may retain most of its original quartz, while, on the 

 other hand, a section with quite fresh feldspar may show much 

 alteration in the quartz. Not uncommonly the alteration pro- 

 ceeds most rapidly along the contact between the quartz and 

 feldspar. It is hardly probable that this results from chemical 

 causes ; it must, rather, be due to the more ready circulation of 

 solutions along these contacts. This may, perhaps, be accounted 

 for by a tendency for quartz and feldspar to separate under 

 mechanical strains. 



The extreme result of the alteration is a mass of the greenish 

 aggregate with no trace of either quartz or feldspar. But more 

 commonly the rock consists of the green aggregate with a 

 greater or less number of quartz grains. 



Under low powers the aggregate has a felt-like appearance, and 

 a green or yellowish color. With crossed nicols it shows aggre- 

 gate polarization of varying intensity, the most thoroughly altered 

 sections being nearly isotropic. 



With higher powers the aggregate is seen to be made up of 

 small, irregular scales, with a single pronounced cleavage. 

 These scales are quite pleochroic in green and yellow, have a 

 parallel extinction, and low double refraction. From these facts 

 it is very probable that the scales consist of some member of the 

 chlorite group, or of one of the nearly related hydrous silicates. 

 An absolute determination of species is out of the question, and 

 it is, moreover, probable that more than one species enter into 

 the composition of the aggregate. Some sections show mingled 



