PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. Ill 



Porphyryte.] 



amygdules. These amygdules are filled with a yellow mineral epidote and a dark 

 mineral chlorite. Some of the amygdules contain only one of these minerals, while 

 others contain both. In the latter case the epidote forms a narrow rim around the 

 outside of the cavity, and within this is usually chlorite, or sometimes chlorite and 

 epidote. At times inside of the rim of epidote is another thin layer of chlorite, and 

 inside of this a mass of epidote. 



Mic. Under the microscope the rock is seen to be decidedly altered. The 

 porphyritic/e/tfopars, on account of their changed condition and the general reddened 

 appearance of the whole section, are sometimes not sharply separated from the 

 groundmass of the rock, when viewed in ordinary light; but in polarized light, they 

 are distinct. They are plagioclase, but their exact place in the series was not 

 determined. The phenocrysts are replaced more or less completely by epidote, and 

 this epidotization of the feldspars is a marked feature of the section. 



The amygdules, as stated above, are filled with epidote and chlorite. The former 

 is in a finely granular condition, while the epidote of the feldspar phenocrysts is in 

 crystalline grains of some size. The chlorite of the amygdules is more characteristic 

 than the chlorite of many of the rocks here described in its marked pleochroism and 

 in the fact that it furnishes a beautiful example of the dark blue interference colors 

 of this mineral. 



The groundmass of the rock is composed chiefly of minute lath-shaped plagio- 

 clases. These are closely matted together and fill up nearly the whole section. The 

 little space in the groundmass not occupied by these feldspars is filled with a 

 confused, fine grained mass of chlorite, epidote, magnetite and apparently a little 

 quartz. In fact, these minerals are scattered throughout the section, and are clearly 

 secondary. The original nature of the groundmass aside from the feldspar is 

 uncertain. The rock is provisionally called a diabase porphyryte, although it is not 

 improbable that it was more in the nature of a trachyte. 



Two sections. Only one, however, was examined, as the other is an inferior 

 section, and there is some doubt about its being correctly labeled. 



Age. Probably a Cabotian eruptive. u. s. G. 



No. 10. PORPHYRYTE. ( Amygdaloidal. ) 



Duluth. Overlies No. 11. 



Kef. Annual Report, ix, page 13. 



Meg. This rock closely resembles No. 9, except that its color is reddish. It has 

 the same porphyritic feldspars and amygdules filled with epidote and chlorite. The 

 epidote, however, is much more abundant than the chlorite. 



No section. 



Age. Cabotian eruptive. u. s. G. 



