644 EBUPTIVE ROCKS FROM MONTANA— MERRILL. vol.xvii. 



somewhat confasing. Geologically they ajipear as one and the same 

 body; from a petrographic standpoint they differ radically. It is use- 

 less to speculate on their possible relationships until further outcrops 

 are found, or until, by digging or blasting, the nature of the inter- 

 mediate zone of decomposed material is made apparent. The total 

 thickness of the eruj)tive sheet or sheets is about 45 feet.* 



Porpliyrite [f). — The upper sheet. Macroscopically, this (No. 38599, 

 U.S.N.M.) is a very tough ard hard dark gray, nearly black, aphanitic 

 rock bearing abundant small pseudo-amygdules of a dull red or yellow- 

 ish green color, and with but rarely a black porphyritic mineral suffi- 

 ciently developed to suggest a member of the pyroxene or amphibole 

 group. In the thin section the rock was found so completely altered 

 that it was only after repeated sections had been cut from samples from 

 various portions of the outcrops that anything like a satisfactory idea of 

 its original nature could be learned. Sections of the freshest samples 

 obtaiuable show under the microscope a nearly colorless devitrified 

 base, impregnated with innumerable small, sometimes meie dust-like 

 particles of opacite and elongated yellowish mica-like needles in places 

 so abundant as to form a truly feltlike groundmass. 



Scattered thickly throughout this groundmass are numerous x^seudo- 

 amygdules of calcite, cliloritic, and ferruginous substances, and occa- 

 sional badly shattered, imperfect, and greatly decomposed augites. 



The amygdules are due wholly to the decomposition of porphyritic 

 augites and olivines, as can be determined by occasional still quite per- 

 fect crystal outlines in the least decomposed portions of the rock, and 

 are in no case true gas cavities filled with secondary minerals. In a few 

 instances the outlines of these cavities were such as to suggest that 

 the decomposed mineral may have been a feldspar, but this can not be 

 determined for a certainty. Sections from the more highly altered por- 

 tions of the rock exhibit interesting changes. The groundmass here 

 (No. 38599, U.S.N.M.), as above, consists of the colorless base so filled 

 with tiie mica (?) needles as to be almost felsitic, but the porphyritic 

 augites are replaced wholly by a light greenish-blue, faintly dichroic, 

 somewhat fibrous hornblende. Although oiitically these secondary 

 forms are undoubted hornblendes giving maximum extinctions on clino- 

 j)inacoidai section of 15°, their outlines, when sufficiently perfect for 

 measurement, are still, in i)art at least, those of augite. In a number 

 of cases the prism outlines on cross sections were measured with 

 results varying from 87° to 89°. The cleavage in such cases was some- 

 what imijerfectly developed, but I was able to obtain measurements of 

 the obtuse angle varying from 123° to 125.5°, which is, perhaps, as 

 close as can be expected in sections cut at haphazard. Although tbe 

 hornblendes are so plainly paramorphic, I have found in no case traces 

 of an augitic nucleus, the change being in all cases complete. . 



Chemical analysis of so highly altered a rock can be regarded as 



* Bull. U. S. Geol. Survey No. 110, pp. 49, 50. 



