136 F. Ward — Lighthouse Granite near N ew Haven, Conn. 



Since the above phenomena are largely the result of dynamic 

 action, they may occur along any fault or fracture zone in this 

 mass. 



Petrography. 



The Main Granite — Megascopically the rock is seen to be 

 a typical granite, i.e., it is composed chiefly of feldspar and 

 quartz and has a granular texture. The feldspar is of two 

 kinds, pink and white, with the pink predominating, and, as 

 usual where the two kinds thus appear together, the pink is 

 orthoclase and the white albite. The quartz is commonly gray 

 and glassy, but in weathered specimens may be coated with a 

 yellowish hydrated oxide of iron. 



Of the accessory minerals biotite is the most noticeable but 

 it is by no means abundant : it usually occurs as scattered flakes 

 evenly distributed, but occasionally it may be seen in smeary 

 segregations or " schlieren." Muscovite appears sparingly ; is 

 more common near Lighthouse Point and in general near the 

 fault contact. Magnetite, though not appearing in every hand 

 specimen, can be said to be a common accessory in a broad 

 w r ay. Garnets have nowhere been encountered : a negative 

 statement of this kind is only of value when the rock is com- 

 pared with the Bran ford granite. The texture is quite uni- 

 formly medium-grained ; there are local exceptions to this 

 general rule. 



Under the microscope the rock is also seen to be a simple 

 granite as far as mineral content goes. The minerals present 

 are, — orthoclase, microcline, plagioclase, quartz, biotite, musco- 

 vite, sericite, chlorite, magnetite, zircon, apatite, (calcite). 



The orthoclase greatly predominates over the other feldspars 

 and presents nothing unusual. Microcline is not present in 

 any great amount ; it shows the usual basket structure. The 

 plagioclase is practically all albite, only a little oligoclase being 

 present. 



The quartz is characterized uniformly by an undulating and 

 broken extinction. In some instances it is so broken as to be 

 granulated. The graphic arrangement of quartz in feldspar 

 occurs in a few cases. 



An occasional flake of muscovite appears in the slides, but 

 most of the white mica is present as fine scales of sericite scat- 

 tered through the feldspars or somewhat segregated in cracks 

 in those minerals. 



Biotite appears as the usual brownish, pleochroic flakes. 

 It may be partially or entirely altered to chlorite. Chlorite 

 when present is only a product of alteration of the biotite. 



Calcite is seen only in those specimens which have been 

 weathered considerably. Magnetite, zircon and apatite occur 



