GEOLOGY OF THE HAYSTACK STOCK 199 



quiescence of considerable duration. At some of these unconformities 

 there is an abrupt change in the appearance of the breccia. There 

 are more or less constant differences between the lower and the 

 upper portions of the breccia, and the series has been divided by Mr. 

 Iddings^ into the acid andesitic and the basic andesitic breccia. 



The acid andesitic breccia. — The acid andesitic breccia consists 

 chiefly of light-colored andesite, dacite, and latite fragments which 

 vary in size from fine dust to masses several feet in longest dimensions. 

 The coarse material is most abundant in the lower portion of the 

 formation, while the upper portion contains beds which are composed 

 almost entirely of volcanic dust. The chaotic basal portion contains large 

 fragments of gneiss and quartzite, which were broken from the pre- 

 Cambrian and Cambrian formations, and probably represent material 

 thrown out at the time of the earliest eruptions. Such fragments are 

 very abundant in the west wall of the Boulder Canyon due west of 

 Haystack Peak. 



The upper portion of the acid breccia is largely composed of 

 fine material; some of it is well stratified but probably of subaerial 

 origin. The bedded tuffs dip at low angles but the dip is not 

 quaquaversal with respect to the Haystack stock. It is probable that 

 the material from these beds came from several sources, and since 

 much of it is very fine, the sources may have been a considerable 

 distance away. The thickness of the lower acid breccia is variable 

 and reaches a maximum of 1,500 feet. It is well exposed above 

 the Cambrian on the divides between the tributaries of the Boulder 

 River at the head-waters of this stream, also south of Haystack 

 Peak at the head of East Fork, and at the head of the drainage of 

 Buffalo Creek. There is an isolated remnant of the breccia cap- 

 ping the gneiss about two miles northeast of Little Haystack Peak. 

 The acid breccia probably once covered a much more extensive area 

 than it does now, and has since been eroded. Mr. Arnold Hague^ has 

 shown that in the Yellowstone National Park this early acid breccia 

 belongs to the Eocene period and corresponds with the Fort Union 

 horizon. The early acid breccia is cut by andesite-dacite stocks and 



1 Iddings, Livingston Folio, p. 6. 



2 "The Age of the Igneous Rocks of the Yellowstone National Park," American 

 Journal of Science, Fourth Ser., 1896, p. 450. 



