ig8 WILLIAM H. EMMONS 



one-half inch to five inches in diameter, cemented by a hmestone matrix. 

 The limestone has been extensively recrystallized since it was deposited 

 and is largely composed of closely interlocking anhedrons of calcite, 

 I ™"^ or less in longest diameter. In some localities it is rich in 

 fossils. On the trail about two miles northwest of the point where 

 Boulder River is intersected by the northern boundary of the area 

 mapped on Fig. i, exposures show that certain beds of the limestone 

 are composed almost entirely of fragments of trilobites. The quartzite 

 is the Flathead quartzite of Mr. Weed^ and the limestone is very 

 probably to be correlated with the Meagher limestone in his section 

 at Helena.^ 



Immediately south of Copper Creek, at a point about three-quarters 

 of a mile above its junction with the Boulder River, the limestone, 

 here thinly bedded and shaly, rests upon gneiss, showing that it was 

 deposited by overlap upon the sinking Cambrian sea bottom. 



The Cambrian beds dip gently west-southwest away from the 

 pre- Cambrian rocks and are intruded by sills of andesite-dacite, 

 and cut by the Haystack stock. At the contact with the stock the 

 limestone is metamorphosed and locally contains secondary quartz, 

 epidote, garnet, and an undetermined amphibole. The attitude 

 of the Cambrian beds at the contact with the Haystack stock is 

 approximately the same as away from it. 



EXTRUSIVE ROCKS 



Occurrence and distribution. — Overlying the crystalline schists 

 and the sedimentary rocks and conforming to the irregularities of 

 their eroded surface is a great thickness of extrusive rocks, consisting 

 of breccias, tuffs, agglomerates, and lava flows. These rocks are 

 formed of material thrown out of volcanic vents and are more or less 

 continuous for many miles to the south and west, covering a vast 

 area in and around the Yellowstone National Park. 



The extrusive rocks do not represent a continuous series, for 

 unconformities of erosion occur at many places. The presence of 

 silicified trunks and stumps of trees, some of them upright as they 

 grew, shows that between the volcanic eruptions there were periods of 



I Livingston Folio, Geological Atlas of the U. S., U. S. Geological Survey. 

 3 Helena Folio, Geological Atlas of the U. S., U. S. Geological Survey. 



