202 WILLIAM H. EMMONS 



are partially concealed by detrital material. In appearance the dike 

 rocks are dark and either aphanitic or contain inconspicuous pheno- 

 crysts of augite, feldspar, and olivine. Under the microscope they show 

 a considerable range of composition. The basalt dikes have a 

 microcrystalline groundmass containing a large number of lath- 

 shaped labradorite crystals which flow around larger phenocrysts 

 of augite and olivine. Hornblende phenocrysts are present in some 

 thin sections. In the less basic dikes, the andesites, the groundmass, 

 sometimes glassy, contains a smaller amount of plagioclase and 

 augite. Hornblende is more abundant than in the basalt dikes. 

 Some thin sections show numerous large andesine phenocrysts. 

 Some of the dikes can be traced into the Haystack stock, though 

 none of them crosses it. Probably many of them are connected with 

 it in depth. 



PETROGRAPHY OF HAYSTACK STOCK 



Relation to other rocks. — The Haystack stock occupies the greater 

 part of Haystack Basin, extending eastward nearly to the East Fork 

 of the Boulder River. A long arm trends westward from Haystack 

 Peak for nearly a mile and crosses the West Fork of the Boulder 

 River; another arm extends northwest to the western spur of Baboon 

 Mountain. Its outcrop altogether occupies an area of about two and 

 one-haH square miles. The stock cuts through the crystalline schists, 

 the Cambrian sedimentary rocks with intruding sills, and the early 

 acid breccia. There is no evidence that the Cambrian beds were 

 turned up by the intrusion, for wherever they outcrop near the stock, 

 their attitude is approximately uniform and they dip southwestward 

 at low angles near the stock in the same manner as at some distance 

 away from it. 



The western arm of the stock is closely related to a fault which 

 extends westward from the north spur of Haystack Peak and crosses 

 the Boulder River at an elevation of about 7,900 feet. North of the 

 fault the Cambrian sediments occur some 600 feet above the stream 

 bed on both sides of the Boulder River. South of the fault the Cam- 

 brian is wanting and the breccia is the lowest formation exposed. 

 The minimum throw of the fault at this point is 600 feet. So far as 

 known, the Haystack magma did not penetrate the shaly beds between 



