142 THE VERMILION IRON-BEARING DISTRICT. 



developed in large quantity. These pre-Cambriaii basic lavas exhibit con- 

 ditions almost exactly the same as those observed by Iddings in the acid 

 lavas of much more recent times in the Yellowstone Park and described 

 by him as follows:" 



111 recapitulation, then, this rhj'olitic lava is a flow about 100 feet thick, except 

 where it has piled up in a small vallej'. It is glass_y, except the lithoidal portion 

 in the valley, and is free from phenociysts. The obsidian is dense in a lower part 

 of the edge and carries numerous spherulites. Large vesicles occur in the upper 

 portion, iind toward the surface of the flow the spherulites disappear and the glass 

 becomes filled with gas cavities and passes up into pumice. * * * These 

 characteristics repeat themselves in the rhyolite in various parts of the park. 



Up to the present time there have been, to my knowledge, only two 

 occurrences of basic spherulites or spherulitic rocks (variolites) described 

 from North Ameinca. The one was by Ransome from California '' and the 

 other by myself from the northern peninsula of Michigan." In the case 

 of the Ely greenstone the spherulitic structure is, as has already been 

 intimated, one of the most common, most charactei'istic, and most striking 

 features of the rock. The spherulitic greenstones are distributed in dis- 

 continuous exposures over a great number of square miles. Jf or instance, 

 they are very common and beautifully developed on the bare hills north 

 of Long Lake in sees. 10, 11, 14, 15, 16, 21, and 22, T. 63 N., R 12 W. 

 They are especially common just southeast of Jasper Lake in sees. 1 and 

 12, T. 63 N., R. 10 W., and sec. 6, T. 63 N., R. 9 W. They are also 

 very common west of North Twin Lake in sees. 10, 11, 14, and 15, 

 T. 63 N., R. 10 W. Well-developed spherulites occur also about a mile 

 north of North Twin Lake at the northeast corner of sec. 12, T. 63 N., 

 R. 10 W. In many of the places mentioned, especially in sec. 6, T. 63 N., 

 R. 9 W., the exposures are almost solid masses of spherulites, which show 

 very beautifully on the weathered surface their radial structure. Figures 

 A and B of PI. Ill are illustrations made from a polished and a weathered 

 specimen, respectively, of these spherulitic rocks, and give a correct idea of 

 their appearance. 



« Geology of the Yellowstone National Park, Descriptive Geology, Petrography, and Paleontology, 

 by Hague, Iddings, Weed, Walcott, Girty, Stanton, and Knowlton: Mon. U. S. Geol. Survey Vol. 

 XXXII, Part II, 1899, p. 365. 



^The eruptive rocks of Point Bonita, by F. Leslie Ransome: Bull. Univ. of Cal., Vol. I, 1893, p. 99. 



«The Crystal Falls iron-bearing district of Michigan: Mon. U. S. Geol. Survey Vol. XXXVI, 

 1899, p. 108. 



