664 ERUPTIVE BOCKS FROM MONTANA— MERRILL. vol. xvir. 



Quartzose hornblende porpliyrite. — Willow Creek at Lower Canon. 

 The eruptive here occurs as an intrusive sheet or boss through the 

 Potsdam quartzites. On the east side of the creek the mass is prac- 

 tically a boss, throwing the quartzites on the north far out of position. 

 Near the canon the mass begins to narrow and passes westward as a 

 broad sheet or dike dipping with the quartzites which appear both 

 above and below. 



In the canon the eruptive is finely exposed in vertical cliffs a.hun- 

 dred or more feet in height and is broken by nearly vertical joints into 

 rudely columnar masses from six to ten feet in diameter. By joints 

 running parallel with the strike the rock is in places also broken into a 

 series of sheets varying from an inch to a foot or more in thickness. 

 East of Willow Creek the main sheet divides, forming two sheets, with 

 the Potsdam quartzites and shales lying between them, and through 

 which has been extruded a brownish coarsely porphyritic audesite. 



The normal rock (No. 62407, U.S.N.M.) is a dense light gray, some- 

 times almost white or faintly yellowish, felsitic to micrograuular mass 

 with inconspicuous phenocrysts and imperfect needle-like hornblendes. 

 No quartzes are microscopically apparent. Occasional bands are 

 apparently holocrystalline, though this variety Avas so badly decom- 

 posed as to crumble and samples fresh enough for study were not 

 obtained. 



In the section the rocks show a very dense microcry stall ine ground- 

 mass ot quartz and feldspar particles, bearing abundant micro-pheno- 

 crysts in the form of dihexahedral quartzes and larger feldspars, a large 

 portion of which are orthoclase, though a few striated forms are occa- 

 sionally seen. The hornblendes, although recognizable macroscopically 

 as fine needles, are scarcely visible at all in the section owing to a decom- 

 position which has given rise to calcite and chloritic products. The 

 only striking feature of the rock is the abundance of the small quartz 

 phenocrysts and their peculiar skeleton-like forms, due to numerous 

 empty cavities and inclosures. Partial analysis on a fresh compact 

 sample yielded : 



SiOi 



K2O 



NajO (by dift'erence) . 



Per cent. 



67.43 

 3.40 

 5.87 



The hornblende andesite mentioned above (No. 62408, TJ.S.N.M.) is 

 macroscopically a somewhat dense, brownish or gray rock thickly stud- 

 ded with white feldspar phenocrysts in all sizes up to ten mm. in greatest 

 diameter. In the thin section it shows a dense feldspar microlitic 

 groundmass with strongly marked fluidal structure, and which bears 

 only the porphyritic feldspars above noted, and numerous badly decom- 

 posed and corroded areas which form their outlines are assumed to have 



