190 THE GEOLOGY OF MINNESOTA. 



[Apobsidian. 



the iron sand to which it gives rise on decay cannot be compared with that of the 

 titanic ores of the gabbro. From considerations given in the chapter on structural 

 geology, it will be seen, however, that the Beaver Bay diabase is to be parallelized 

 with the gabbro at Duluth and at Short Line Park, and that these are the surface 

 representative of the great "basal flow" from the gabbro batholyth, and would neces- 

 sarily partake of its ferriferous character. N. H. w. 



No. 127. APOBSIDIAN. 



" From near the mouth of the river at Beaver bay. A ruetamorphic rock presenting another condition of 

 No. 124; frequently jointed, breaking so easily along predetermined planes that it falls, under the hammer, into 

 small fragments, making it difficult to get a fresh fracture. In the main it is slaty, but its texture is tough and 

 its exterior is angular. It is ashen gray, but has, between the laminations, thinner lighter laminations of appar- 

 ently siliceous matter; suddenly rises in a knob and disappears under the drift. In color, structure and texture 

 this differs from any rock before seen on the shore. It rises about sixty feet and extends about 120 feet. Micro- 

 scopically it appears to consist of quartz in fine grains, in a noncrystalline base. It extends more or less back 

 from the mouth of the creek, toward the west, and appears slightly on the other side of the creek (see No. 528)." 

 (Ninth Annual Report, pages 32, 33.) 



Kef. Annual Report, ix, pages 32, 33, 39, 53; Annual Report, x, pages 41, 112, 113, 141; Annual Report, xiii, 

 page 100 (No. 156), 103; Proceedings of the American Association for the Advancement of Science, vol. xxx, page 

 163; Bulletin viii, page xxxiii; U. S. Geol. Survey, Mon. 5, page 107 (No. 790). 



An aphanitic, ashen-gray rock, hard and siliceous. It is crossed by short 

 bands of a lighter color which rather suddenly cease, and are sometimes bent. These 

 bands are an eighth of an inch, or less, in width, and are white or pinkish except for 

 a narrow central line which is darker colored and seems to be mostly quartz. No 

 crystalline grains can be seen in the rock. 



Mic. In ordinary light nothing can be seen but a colorless, transparent and 

 structureless groundmass in which are minute grains of hematite and magnetite and 

 small, gray, semi-opaque cloudy areas, with an occasional small rounded greenish or 

 yellowish grain appearing like epidote. Under polarized light the groundmass 

 breaks up into small, irregular, not sharply outlined areas of quartz which include 

 the other minerals of the rock poikilitically. The semi-opaque, cloudy areas are 

 supposed to be feldspathic, but they are very minute and are practically isotropic, 

 so they cannot be determined definitely. In places these poikilitic areas of quartz 

 are larger than common, but nowhere do they attain the size of the areas shown in 

 No. 68; in fact, No. 127 would hardly be supposed to be poikilitic when certain parts 

 of the section were examined under a low power. One of the sections is cut across 

 one of the bands which are so prominent a feature of the hand sample. The band 

 appears simply as a clearer line which is made up of quartz grains and some of the 

 iron ore and cloudy grains of the mass of the rock. Nothing can be seen to show 

 why the borders of these bands appear so distinct in the hand specimen. The bor- 

 ders appear exactly like the rest of the section, although there is perhaps a slightly 

 greater accumulation of the cloudy material of the groundmass. 



Two sections. 



