PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 125 



Diabase.) 



the hand specimen. Sometimes these red and yellowish elements are united in the 

 same spot, the former surrounding the latter, and occasionally the red spots show 

 distinctly a twinning on the albite plan. 



Mic. The rare porphyritic feldspars are much decayed, showing quartz and 

 hematite, and apparently calcite, as resultant minerals, the iron probably derived 

 from the surrounding matrix, yet they are distinctly striated in some places and 

 undoubtedly they resulted from the cooling of a molten basic magma. Throughout 

 the rock are also microlitic plagioclases. 



The augitic element is so changed that it cannot be identified as such, but the 

 products of change from augite are the only evidence of its former existence (see 

 No. 27). 



That olivine was originally in the rock, is shown by the roundish green chloritic, 

 isotropic areas, crossed by the ferruginous bands that indicate the original cleavage 

 of that mineral. These areas crowd on and displace the crystalline boundaries of 

 the feldspars, showing their earlier date. 



A micro-chemical test was made on the red element, not showing definite crys- 

 talline characters, and the only result was the appearance of fluosilicate of lime with 

 very rare hexagonal rods of the same salt of soda. The conclusion to be drawn from 

 these facts seems to be that the feldspar was a lime-soda plagioclase. In decay it 

 has given up its soda, its lime has given rise to calcite, and, with the formation of a 

 little quartz, the forms of the plagioclases have been lost, a red stain involving not 

 only the remnant of the plagioclase itself but spreading irregularly through the sur- 

 rounding rock, thus converting a porphyritic feldspar into irregularly shaped areas 

 of a red color. 



One section examined. 



Age. Cabotian. 



Remarks. This rock seems to be one of the series of eruptives extending along 

 the shore eastward of Minnesota point, mingled with amygdaloidal and tuffaceous 

 material. The red stain, which usually is evidence of the proximity of some of the 

 elastics of the Animikie and of the fusion, or at least the wide dissemination of some 

 of their elements in the basic eruptive, cannot certainly be attributed to that cause 

 in this rock. Yet it is also equally impossible to exclude that agency in the change 

 of these feldspars a change that took place, probably, mainly while the rock was 

 yet hot, a period during which both gas and water must have permeated the erup- 

 tive, carrying out such chemical alterations as were fittest to the nature of the 

 attacking agent. N. H. w. 



