GEOLOGY OF THE HAYSTACK STOCK 201 



Under the microscope the groundmass is seen to be microcrystalline 

 and contains phenocrysts of andesine, green hornblende, quartz, 

 orthoclase, magnetite, and biotite. Calcite and serpentine are present 

 as secondary minerals. Orthoclase and quartz are sometimes 

 present in considerable amount when the composition of the rock 

 approaches that of quartz-monzonite-porphyry. Since the sills 

 are cut by the Haystack stock they are older than it, and since they 

 do not cut the breccias they are possibly older than them, and pre- 

 sumably the oldest Tertiary volcanics in the district. 



Andesite-dacite stocks. — Andes ite-dacite stocks cut the pre- 

 Cambrian rocks and the basic breccia. The largest one is about two 

 miles east of Little Haystack Peak, and covers an area of nearly 

 two square miles. Several smaller bodies occur along the southern 

 border of the area mapped." 



The andesite-dacite is gray or pinkish gray, and contains pheno- 

 crysts of feldspar, hornblende, quartz, and mica. Under the microscope 

 the groundmass is seen to be microcrystalline and contains pheno- 

 crysts of andesine, quartz, green hornblende, pyroxene, orthoclase, 

 and magnetite. The andesite-dacite stocks are yoimger than the 

 early breccia and probably older than the later breccia. They are 

 closely allied to the sills in composition, but if the sills are older than 

 the early breccia, which the stocks cut, a considerable period intervened 

 between the two intrusions. The composition of the andesite-dacite 

 is near that of the early breccia, and the sills, stocks, and breccia 

 probably came from the same or closely related magmas. 



The Haystack stock. — The Haystack stock is an intrusive body 

 of irregular shape cutting through pre-Cambrian schists, Cambrian 

 sedimentary rocks, and the early acid breccia. It is composed 

 entirely of granitic rocks varying in composition from quartz-mon- 

 zonite to olivine-gabbro. These grade into one another, and it is 

 assumed that they represent the products of differentiation from a 

 common magma. 



Basalt and andesite dikes. — Basalt or andesite dikes cut all the 

 other rocks. They are most abundant in the country south, west, 

 and east of the Haystack stock, and have a rudely radial arrangement 

 around it. In width they vary from four inches to forty feet and 

 some are exposed for considerable distances, though most of them 



