Keid.] 



The Igneous Rocks near Pajaro. 



181 



THE ACID DYKES. 



The two rocks already described are cut by a network of small 

 dykes, the rocks composing which range from a fine grained 

 aplitic type to a very coarse grained pegmatite. Intermediate 

 types are also found, as will be discussed later. 



Aplite. — In the hand this type is of fine grain, showing clear 

 quartz, a white or pinkish cloudy feldspar, and a small amount 

 lustrous muscovite. The quartz and feldspars appear to be in 

 grains .7 mm. in size, and some of the muscovite flakes are 

 2 mm. in diameter. The mica, while normally of small amount, 

 in some cases increases so as to make up practically all the rock. 

 Under the microscope the rock is seen to be composed of quartz, 

 orthoclase, microcline, plagioclase, and muscovite, with a very 

 few ragged crystals of hornblende. A few grains of pyrrhotite 

 occur, usually much altered to hematite. Epidote and chlorite 

 occur secondarily derived from the hornblende. 



The quartz occurs in irregular grains molded around the other 

 constituents, and is clear and free from inclusions. It makes 

 up about 25 per cent, of the rock. In size of grain it averages 

 .36 mm., with a maximum of 1.46 mm. The orthoclase occurs 

 in slightly larger crystals, elongated parallel to the clino-axis 

 and exhibiting some very good cleavages on the base (001) and 

 (010). The size of the crystals averages .75 nun., with a maxi- 

 mum of 2.95 mm. It is much decomposed, zonal structure being 

 often well developed in the alteration. Kaolin is the chief product 

 of decomposition, though some sericite is formed, the crystals 

 altering from the center outward. The orthoclase often contains 

 inclusions of flakes of muscovite, seemingly parallel to the 

 crystallographic directions (001) and (101). Wavy extinction is 

 frequent. The orthoclase makes up between 40 and 50 per cent, 

 of the whole. A small amount of microcline is found, usually in 

 smaller grains than the orthoclase, recognized by its character- 

 istic cross-hatched structure. It crystallized out before the 

 cpiartz, coincident with the orthoclase. It makes up about 5 per 

 cent, of the rock. 



The plagioclase occurs in irregular crystals, of the same size 

 as the potash feldspar. It is twinned only on the albite law. 



