OF ARTS AND SCIENCES. 281 



zone 0: it, indicate that the feldspar is either labradorite or anorthite. 

 A large part of these crystals show an aggregate polarization so closely 

 resembling that of the prehnite pseudomorphs after plagioclase, that 

 there is little doubt that they are partly changed to prehnite. The 

 change to chlorite is evident, in all stages, from chlorite disseminated 

 through the plagioclase, to fiuished pseudomorphs and pseudo-amyg- 

 dules. 



The analcite bodies are seen to have no true amygdule-walls, but 

 to penetrate the matrix in the most irregular manner, as in pseudo- 

 amygdules, which they really seem to be. They are traversed by 

 seams of chloritic substance, and are often changed into this to a con- 

 siderable depth on the sides, besides containing in places groups of the 

 same chloritic substance in the interior. They are also cut by seams 

 of calcite, which also traverse the matrix. The chloritic substance 

 seems in places to have formed finished pseudomorphs after the anal- 

 cite. The chloritic substance, resulting from change of analcite, is less 

 clear, and much finer textured than that resulting from the change of 

 feldspar. 



There are no visible traces of pyroxene. It seems to be quite prob- 

 able that the analcite is derived from the plagioclase, but I have no 

 data for determining its relative age as compared with that of the 

 prehnite. As a rule, the secondary alkaline silicates form only in 

 the amygdaloids proper, and in veins. 



(I.) ]. Plagioclase. 



(Labradorite or Auorthite.) 



1 . 



(II.) Prehnite after plagioclase. Analcite. 



\ \ . 



Chloritic substance after prehnite. Chloritic substance after Analcite. 



Bed No. 22, of the Eagle River Section, has forty feet of lower zone, 

 and sixteen of pseudo-amygdaloid. Thin sections of the diabase from 

 the lower zone show the plagioclase crystals mostly very fresh. Optical 

 measurements on sections in the zone : u, indicate that the feldspar 

 is labradorite. They are closely surrounded by the younger pyroxene, 

 which is in part very fresh, but in places is changed to its characteris- 

 tic pseudomorphs. This secondary mineral is better marked here than 

 is usual ; it has well-defined, perfect cleavage, strong absorption for in- 

 tensity, being dark when the cleavage lines coincide with the shorter 

 diagonal of the nicol. Between crossed nicols, darkness occurs when 

 the cleavage lines are parallel to a nicol plane. I observed no instances 

 in which it revolved dark. These characteristics indicate an ortho- 



