306 PROCEEDINGS OF THE AMERICAN ACADEMY 



Epidotic Amygdaloid. — In the copper-beai-ing beds worked at Port- 

 a«Te Lake, there occur frequently considerable masses of a hard, light- 

 green, generally fine-grained, epidotic rock, which is often quite free 

 from amygdules. Thin sections show that it consists of a granular 

 ao-gregate of epidote and quartz, with metallic copper. In parts of the 

 section, the epidote predominates, and the quartz is only just apparent. 

 In other parts, the quartz is in the majority. The grains forming the 

 quartz aggregate are small, and not detrital ; they are united without 

 interstices, and were undoubtedly crystallized in the positions they 

 now occupy. The quartz also forms veins, cutting through the more 

 epidotic portions, and throughout the sections it is clearly younger than 

 the epidote ; for grains and crystals of the latter are included in the 

 interior of quartz individuals. It is evident that either the two min- 

 erals crystallized together, or that the ejjidote was originally held 

 together by some substance which has been replaced by <]uartz. The 

 observations on thin sections of the quartz-epidote rock from the 

 Huron justify us in supposing that here, as there, the quartz has 

 replaced some more soluble mineral. On one part of that specimen 

 we saw remnants of prehnite, partially changed to calcite, and also 

 half-finished pseudomorphs of quartz after the same calcite. In an- 

 other part of that specimen the calcite was gone : but the quartz form- 

 ino- nearly all of the matrix enclosed the same alteration products of 

 prehnite that we saw in the calcite in the other part; while in the 

 epidotic part of the specimen we had simply epidote added as an older 

 constituent, enveloped by quartz. 



All my observations seem to indicate clearly, that the epidotic por- 

 tions of the altered amygdaloids are a product of the alteration of 

 prehnitized matrix. 



Its paragenetic relations are, probably, nearly as follows : — 



(I.) Prehnitized Matrix. 



(11.) 



Epidote. 



(III.) Calcite enveloping Epidote and other products of prehnite. 



(IV.) Quartz replacing Calcite. 



I have attempted to correlate the typical ones among the different 

 sets of paragenetic schemes in the annexed table, and to show, by this, 

 which portions of the progressive changes caused the pseudo-amygda- 



I 



