308 PROCEEDINGS OP THE AMERICAN ACADEMY • 



This is the broader history. Orthoclase is here, as in the pseudo- 

 amygdaloid, of sporadic occurrence, and a product of the prehnite. 



The changes under II. may affect only the amygdules, or, if the 

 matrix was prehnitized, it applies to the whole mass of the amygda- 

 loid. It does this in such a manner that, where carried to its extremes, 

 considerable portions of the bed have lost every semblance of an amyg- 

 daloid, and consist now of chlorite, epidote, calcite, and quartz, more or 

 less intimately associated, or forming larger masses, of the most indefi- 

 nite shapes, and merging into each other. Sometimes portions of jjar- 

 tially altered prehnite occur. In places, considerable masses of rich 

 brown, and green fresh prehnite filled with copper occur ; but, as a rule, 

 this mineral has given way to its products. 



To this process, the copper-bearing beds of Portage Lake — wrongly 

 called lodes — ov^e their origin. Considerable portions of these beds 

 are but partially altered amygdaloids, containing amygdules of preh- 

 nite, chlorite, calcite, or quartz, with more or less copper ; other portions 

 are in the condition described above. 



This, too, (II. and III.), appears to have been the principal period of 

 concentration, of the copper. In the still amygdaloidal portions, this 

 metal was deposited in the cavities and in cleavage-planes of some 

 minerals, and replaced calcite amygdules, &c. But in the confused 

 and highly altered parts of the bed it crystallized free, where it had a 

 chance : more generally it replaced other minerals on a considerable 

 scale. It formed, in calcite bodies, those irregular, solid, branching 

 forms, that are locally known as horn-copper, often many hundred 

 pounds in weight ; in the epidote, quartz, and prehnite bodies, it occurs 

 as thread and flake-like impregnations ; in the foliaceous lenticular 

 chloritic bodies, it formed flakes between the cleavage planes and 

 oblique joints, or in places — and this is more particularly true of the 

 fissure-veins, which we are not now considering — it replaces the chlo- 

 ritic, selvage-like substance till it forms literally pseudomorphs, some- 

 times several hundred tons in weight. 



When the amygdaloid has arrived at the condition we have been 

 describing, it assumes some of the characters of a vein, in that, although 

 it presents no open fissure, it contains greater or smaller masses of 

 calcite and other minerals that are easily replaced by an intruder. To 

 this period, probably, belongs the replacement of calcite by datolite ; 

 and here, also, the rather rare occurrence of analcite crystals, and the 

 pseudomorphs of oi'thoclase after these. 



As I have already remarked, the pseudo-amygdaloids are merely 

 altered forms of the same rock as the lower zone. There seems to be 



