414 THE GEOLOGY OF MINNESOTA. 



[Sandstone. Basalt. Quartzyte. 



No. 546. SANDSTONE (or arkose). 



Grand Portage island. Eight feet thick. 



Ref. Annual Report, x, pages 46, 47. (Compare No. 25G.) 



A very fine-grained, purplish gray sandstone. Along one edge of the 

 specimen is a band, about half an inch wide as far as shown, of an ashen gray color. 

 This band seems to be of the same grain and composition as the rest of the rock. 



Mic. The section is composed of small grains of quartz and feldspar, with a 

 small amount of yellowish brown material between some of the grains. The grains 

 are rounded or sub-angular and are closely crowded together. Some of the quartz 

 grains show secondary enlargements and it seems probable that many more than 

 show this feature distinctly have also been enlarged. The feldspar is largely cloudy 

 and probably orthoclase, although a few plagioclase and less microcline grains are seen. 



One section. 



Age. Potsdam. u. s. G. 



No. 547. BASALT. (/Arkclyte.) 



Grand Portage Island. A bed of trap. This is sometimes brecciated or finely and irregularly jointed, with 

 white nodules of saccharoidal calcite. Resembles rock No. 541. Probably an intrusive, thirty-six feet thick. 

 Ref. Annual Report, x, pages 46, 47. 



Meg. Heavy, fine grained, nearly black. 



Mic. The slide is characterized by porphyritically distributed small attgites, 

 lying in a glass;/ and microlitic base. The augites are somewhat radially grouped, 

 as if twinned on one of the domes. There is but little of the non- crystallized magma. 

 Magnetite in small cubes is frequent. Some microlitic feldspar and considerable 

 calcite, with pale yellowish areas of a very finely fibrous alteration mineral, which is 

 the same as mentioned in No. 544 as possibly tJialitc, are the only noteworthy 

 features. The rock is similar to No. 544. 



Two sections. 



Age. Manitou. N. H. w. 



No. 548. QUARTZYTE. 



Grand Portage island. Five feet thick. (Compare No. 256.) 

 Ref. Annual Report, x, pages 46, 47, 48. 



Meg. Sometimes with mica scales between the beds, of a dark color, but 

 generally striped with brown-red, with thin laminations. In some parts it is gray. 



Mic. The rock is composed of angular quartzes and feldspars, some of the latter 

 being of.playloc/tmc, and of devitrified ylasx, the last constituting about one-half. 

 Other substances are present, but it is not possible to determine them, except 

 lii-iitatite and calcifc. There are some light-brown grains that may be of 



One section. 



Age. Potsdam. 



