788 THE GEOLOGY OP MINNESOTA. 



[Marble. Conglomerate. Gabbro. 



the continued growth of the hornblende was free from the effect of the augite, and 

 took a more perfect hornblende color. All the irregularities of coloration are 

 explicable on this hypothesis, there being allowance made for irregularities and 

 imperfections in the original augites. If this be a correct interpretation, it would 

 follow that such contrasts of color, called " frayed " ends, by G. H. Williams, due to 

 supposed dynamic action, and " secondary enlargements," by Van Hise, are only 

 evidence that the hornblende is derived from an original augite crystal or fragment of 

 a crystal, and that there was but a single period of growth. N. H. w. 



No. 1792. MARBLE. 



Near the same place as the last. 

 Ref. Annual Report, xxii, page 5. 



Meg. In the conglomeratic portions of the greenstone, particularly in the 

 matrix surrounding the boulders, is seen coarse quartz with calcite, mingled in which 

 are also isolated pieces of some green shale or scale. 



Mic. The slide only contains granular calcite, a kind of marble. One section. 



Age. Archean (Keewatin). N. H. w. 



No. 1793. CONGLOMERATE. ( Irony pebbles. ) 



Virginia. 



Ref. Annual Report, xxii, page 6. See, also, chapter on the Virginia plate in volume iv. 



Meg. " Basal conglomerate, from the bottom of the supposed preglacial gorge, 

 at Virginia. This is of ferruginous pebbles, mingled with dust and dii-t of the iron- 

 bearing rocks. It may be of Cretaceous origin. It has a later infiltration of white 

 silica which now forms its principal cementing bond." No section. 



Age. Probably the basal beds of the Cretaceous. N. H. w. 



No. 1797. GABBUO (with orthoclase). 



Duluth. 



Ref. Annual Report, xxii, page 6. Compare Nos. IB, 5, ete., 854G and 854(A)G. 



Meg. The rock in general contains numerous small masses of augite syenyte, 

 and is therefore accompanied by numerous crystals of orthoclase. 



Mil. The section shows both rocks. Along with reddish orthoclase, in one 

 portion, are quartz and hornblende, with idiomorphic apatites of large (microscopic) 

 dimensions, which show distinctly two cleavages, viz.: that transverse to the crystals 

 and that parallel. The other part consists almost wholly of a single feldspar 

 crystal. This also contains similar apatite. It is approximately parallel to 010, 

 having n f obliquely visible in the interference figure, and showing scant traces of 

 albite maclation, as well as pericline. Its extinction is at 10 on the albite lines 

 (/. c., the cleavage 010) and 55 on the pericline. Neither cleavage is observable, as 



