PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 107 



Diabase.] 



possible, however, that the hornblende is original, and that, with the exception of 

 the epidote and chlorite, the groundmass is now not much altered from the condition 

 in which it solidified; in such case the rock might be called a dioryte porphyryte, 

 and it is similar to some of the rocks described by Iddings, from the Yellowstone 

 National park.* It, however, seems more probable that the groundmass is consid- 

 erably altered from its original condition, which might have been an aggregate of 

 lath-shaped plagioclase (still remaining) and augite, making the rock a diabase 

 porphyryte, to which species it is here provisionally referred. u. s. G. 



Age. Perhaps Cabotian. 



Remark. This rock seems to be from a series of very old eruptives, which at 

 Duluth and elsewhere are cut by a series of later dikes, and are associated with the 

 red rock series. N. H. w. 



No. 7D. DIABASE. 



Duluth. Prom a dike near the lake running toward Minnesota point, and apparently extending under 

 the point. 



Ref. Annual Report, ix, page 12. 



Meg. A tine-grained, dark greenish ophitic diabase. 



J//r. The feldspar preceded the pyroxenic element in date of crystallization. 

 It is uniformly lath-shaped, with more or less tapering extremities, and is less affected 

 by later alteration than the pyroxenes. It composes about three-fourths of the 

 entire rock. 



The pyroxene is almost entirely altered to a greenish chloritic mineral, with 

 simultaneous generation of much magnetite, there being only a few remnants which 

 yet polarize distinctly. The alteration product, aside from the magnetite, consists 

 of a confused mesh of chloritic shreds that overlap each other and give a character- 

 istic felted polarization between crossed nicols. So far as can be seen, the magnetite 

 is entirely of secondary date, taking position in the interstices of the original 

 pyroxenes, and occasionally developing also their forms. The relation of the 

 pyroxene to the earlier feldspar, areally, is that of a mosaic, rather than of ophitic 

 structure. The forms of these minerals are not perfected, the feldspar because of 

 the interference of many cotemporary growths, resulting in a crowded crystalline 

 mass, the latter because of the later generation which allowed them only the 

 interstices between the feldspars for a field in which to develop. 



The magnetite and chlorite very probably originated during the cooling stage 

 of the rock (see Part III). 



Age. Perhaps Manitou. N. H. w. 



* Tn-i-t/th Annual Report U. S. Geol. Survey, pp. 569-661,1891. 



