PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 127 



Sandstone. Diabase.] 



No. 30. SANDSTONE. (Pyroclastic.) 



Duluth. Rock next west of Mallmann's dike, east of the old Fishery. Extends 400 feet. 



This rock shows non-conformable stratification or cross-bedding, perhaps separated by igneous eruptions 

 and lava flows in other places. One part dips but little, and the other part dips east 30 south, and in amount 

 about 15 or 20. It varies from a brownish, siliceous sand rock to one that is greenish and aluminous. 



Ref. Annual Report, ix, pages 15, 20; Annual Report, x, page 36. 



Meg. A thinly bedded, grayish or greenish rock, of homogeneous grain and 

 aspect. 



Mic. The section shows a fragments! structure, and a composition mainly of 

 particles referable to eruptive agencies. These grains consist of quartz, titaniferous 

 magnetite (changed to leucojcetie), plagioclase, epidote, x/ilinir, and devitrified glass. 

 These are cemented together by chlor/fc, i-ii/i-i/r and probably some hematite. There 

 are a few grains that have the appearance of having been derived as particles from 

 a glassy rock. 



Two sections. 



Age. Cabotian. 



Remarks. This rock is probably of sedimentary origin, even by erosion. There 

 cannot be distinguished any lapilli, as a product of explosive action, but the particles 

 consist almost entirely of mineral fragments frequently rounded as by beach 



friction.* N. H. w. 



No. 31. DIABASE (with olivine). 



Duluth; from Mallmann's dike . 

 Ref. Annual Report, ix, page 15. 



Meg. Fine-grained, nearly black. Similar to Nos. 7D and 15. 



Mic. Ophitic relation of the feldspars to the augite (or the devitrified glass) is 

 conspicuous at a glance. They are twinned, lath-shaped, and although more 

 stained with hematite than the feldspars in No. 15, their large extinction angle 

 (about 28) indicates that they are probably labradorite. Sections cut parallel with 

 the tablets (i. e., about parallel to 010) exhibit sometimes a zonal or shelly structure. 



The augite (?) is entirely changed to a greenish substance, with accumulation 

 of much magnetite. It is impossible to affirm what portion of this ophitic substance 

 was originally augite perhaps none of it. It appears that in some cases, after the 

 crystallization of the feldspars, the rest of the magma never gave birth to augite, 

 but, being consolidated in a more or less glassy condition, has afforded products that 

 are undistinguishable from altered augite, and this may be the case in many others 

 of these rocks. 



Olivine, though giving a product very similar to that of augite, yet shows the 

 following differences: (1) The earlier original independent crystalline form is apt 

 to be preserved. (2) The magnetite accumulates along the original cleavages, or 



Compare "Volcanic ash from the north shore of lake Superior," by N. H. WINCHEL.L, and U. 8. GRANT. American 

 Geologist, vol. xviii, pp. 211-213. October, 1896. 



