OF ARTS AND SCIENCES. 301 



teristic chloritic mineral ; and others formed of granular aggregates of 

 epidote and quartz are very frequent. Many of the plagioclase crystals 

 seem to be partially changed to epidote. 



Although no prehnite was observed in these specimens, the whole 

 mode of occurrence seems to point to an alteration of prehnite as the 

 starting-point for the formation of these tertiary products. 



Paragenesis: 1. Pseudo-amygdaloidal chlorite ; 2. Epidote and ortho- 

 clase ; 3. Quartz. 



Amygdaloid from Huron Mine. — In the copper-bearing amygdaloid 

 beds of Portage Lake occur frequently considerable masses of amyg- 

 daloid, in which the matrix is a coarse, irregular patch-work of mas- 

 sive chloritic substance, of quartz, and of epidote. I have before me a 

 specimen, six inches square, containing all these varieties. One end 

 has a soft, dark-green, chloritic matrix, with large patches of prehnite, 

 more or less altered to soft, light-green substance, and to calcite. 



Thin sections show that the matrix has been wholly prehnitized. 

 Fragments of prehnite remain throughout the matrix ; but it is mostly 

 changed to a soft, yellowish-greeu substance. 



The amygdules, which were also of prehnite, are now, in places 

 partly, in others wholly, changed. Many of the amygdules consist 

 novv partly of calcite, which, from the manner in which it encloses 

 particles of prehnite, and of the soft, yellowish-green product of preh- 

 nite, is evidently pseudomorphous after it; the other portion of the 

 same amygdule is often prehnite, more or less changed to a mass of 

 prisms of a light-green substance, which form both on the outside and 

 in the interior of the amygdules. These prisms are monoclinic, and 

 are, probably, epidote. While the calcite encloses the yellowish-greea 

 alteration product, it does not contain this epidote-like substance. 

 "Where this is in contact with the calcite, the line of separation is 

 sliarply drawn. The calcite was formed contemporaneously with the 

 yellowish-greeu product, or before it; it forms also veinlets through 

 the matrix. 



After part of the prehnite had been replaced by calcite, this form of 

 change ended, and the remaining prehnite was subjected to a new 

 process of alteration, — the change to the prismatic substance. In 

 other amygdules a still later phase is apparent ; here, after a part of the 

 prehnite had been changed to the yellowish-green substance and cal- 

 cite, and the rest to the soft, green, prismatic substance, tlie calcite was 

 replaced by quartz. The quartz forms veinlets in the matrix, cutting 

 those of calcite, while in the amygdules it encloses many fringed-edged 

 fragments of calcite, and also the epidote product of prehnite. The 



