Reid.] 



The Igneous Rocks near Pajaro. 



183 



Under the microscope the quartz and the feldspar are seen to 

 be intricately crystallized in interlocking grains. As with all the 

 other rock types discussed, the grain is finer than appears in a 

 hand specimen. 



The quartz is the last mineral to separate out, occurring 

 in large irregular crystals molded on and around the other 

 constituents, It shows undulatory extinction an 1 also a 

 peculiar faint irregularly lined structure between crossed nicols 

 that looks somewhat like an incipient cross-hatched structure of 

 mieroline. This structure is no doubt due to strains in the rock. 

 Inclusions in the quartz are few. It makes up from 30 per cent, 

 to .">() per cent, of the rock, varying in different parts. The feld- 

 spars are orthoclase, microcline and plagioclase. These are asso- 

 ciated in a rather complex manner, the relative proportions of each 

 varying in different portions. The orthoclase, however, averages 

 less than in the aplite. It shows no twinning, but cleavages on 

 (001) and (010) are very well developed, that on the base being 

 the better. It is somewhat decomposed into cloudy kaolin, but 

 most of its opaque appearance is due to a great number of 

 inclusions. These are small flakes of muscovite, with many 

 liquid inclusions, and smaller ones that cannot be definitely deter- 

 mined. The orthoclase makes up about 20 per cent, of the rock. 

 The plagioclase shows the usual albite twinning, with a few peri- 

 cline lamellae. The usual optical methods show it to be 

 oligoelase, as in the aplite. It shows less inclusions and products 

 of decomposition than the orthoclase, with much poorer cleavage. 

 A little calcite is formed secondarily. Checks on the determina- 

 tion of the feldspars were made by specific gravity tests. By 

 Klein's solution the orthoclase, with good cleavage and very 

 cloudy appearance, came down at 2.57, and the clearer plagio- 

 clase, with poor cleavage, at 2.63. Also microcline, with its 

 usual characteristics is present sparingly. It appears to have 

 crystallized before the quartz, at the same time as the orthoclase. 



The biotite is almost all altered to chlorite in the freshest 

 rock to be obtained. At a later date, when the fresh rock is 

 exposed in the quarry face, further study may be profitable. 

 Hornblende occurs very sparingly as an accessory in small irreg- 

 ular grains much altered to epidote. Its characteristics are the 



