PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 145 



Basalt.] 



feldspars replaced by chlorite, although we have uo complete section of one of these 

 dark areas. 



Two sections. 



Age. Cabotian. 



lienturk. The structure of this rock allies it with the diabases. The quartz is 

 regarded as secondary. In almost every particular this rock resembles No. 7. An 

 analysis of No. 7 is given and there are also some remarks concerning the origin and 

 nature of the rock. In the field this rock disintegrates into small angular pieces 

 according to innumerable weather joints. u. s. G. 



No. 52. BASALT (or zirkelyte). 



Duluth. Underlying No. 51, to which there is, in the iield, a gradual transition, and overlying No. 53. 

 This rock makes three heavy layers, or beds, each bed being finely and closely basaltiform, the columns being 

 set so as to constitute layers running west 10 north, and east 10 south, from an inch to two inches in thickness, 

 in position perpendicular to the heavy bedding. They were doubtless produced by the baking effect of No. 53, 

 which comes on suddenly. 



Ref. Annual Report, ix, pages 18, 19; Annual Report, x, pages 109, 110. 



Apparently a dense, black diabase, contacting with cooler rocks. There 

 is great uniformity of aspect in this rock. It shows in some parts a finely dissem- 

 inated microlitic feldspar, and in others such feldspar is wanting entirely. It has 

 much pyrite in isolated fine specks, and occasionally in form of a thin seam. 



Mir. Sections made from this rock differ somewhat in fineness of grain, and 

 in the amount of yiiarfz present, but they are essentially the same. In some of the 

 sections the feldspars are quite well formed, though small, the angite can with some 

 uncertainty be identified, and the nun/iniit/' and spongy pyrite are abundant, par- 

 ticularly the former. There is a brown amorphous and isotropic glassy remnant of 

 the magma which in the angles between the feldspars acts ophitically about them. 

 r/titlofc is gathered sometimes, in these coarser portions of the rock, in the pseud - 

 amygdaloiclal areas, and in other small areas quartz appears. Apatite crystals pierce 

 the section in all directions. 



Other thin sections evidently come from more glassy portions. The feldspar 

 crystallites are identifiable only occasionally as straight openings amongst the aggre- 

 gated globulites of magnetite and augite(?) or olivine(?). When the section is highly 

 magnified the magnetite globules appear crowdedly immersed in the glassy matrix, 

 and occasionally are ranged so as to form fine thread-like crystallites. 



Between crossed nicols the sections are nearly dark constantly, owing to the 

 prevalence of the glassy magma. 



Three sections. 



Age. Cabotian. 



JlfinnrL: Nos. 50, 51 and 52 are allied in the field and in petrographic charac- 

 ters. The first and the last are coarser grained, and have much more quartz. No. 



11 



