Reid.] 



The Igneous Rocks near Pajaro. 



179 



being limited to magnetite and apatite, and secondary products 

 are practically absent. 



The biotite shows its usual characteristics. It is red-brown 

 in basal sections, with strong pleochroism in sections perpendicu- 

 lar to the base, showing colors from yellow to deep brown. 



The magnetite is a striking constituent of the rock, occurring 

 in large irregular grains up to 1.01 mm. in size. Also it shows 

 well developed crystal form in octahedra. Many of the crystals 

 show striatums which may be traces of cleavage, and a brilliantly 

 reflecting surface of a crystal was observed in one slide. The 

 percentage of magnetite in the rock, as extracted by the magnet, 

 is 2.05 per cent. The apatite, in prisms .19 mm. by .045 mm. 

 m diameter, and grains of pyrite in all sizes up to 1 mm. occur 

 in the usual manner and need no detailed description. 



The last material to crystallize is some interstitial quartz, 

 sometimes in relatively large grain 2 mm. in size. It is not 

 abundant, and when found, has the usual characteristics of 

 granitic quartz. The minerals in contact with it often exhibit 

 well developed crystal faces. 



In structure the rock cames nearest to the granitic, or hyp- 

 idiomorphic granular, although showing traces of porphyritic. 

 The order of crystallization is, from first to last, magnetite, apa- 

 tite, hornblende, feldspar, and quartz. 



THE BASIC PHASE. 



Iu the hand this rock appears made up of black hornblende 

 and glassy feldspar, of a grain equal to that of a medium fine 

 grained sandstone, the crystals being .5 mm. to 1 mm. in size 

 The dark mineral appears in slight excess, causing the rock to 

 show a very dark color. In the field this variety occurs in 

 inclusions in the main mass from an inch in diameter to large 

 irregular lens-shaped bodies many feet in diameter. These larger 

 occurrences stand nearly vertical and are invaded by small 

 intrusions of the main mass. Some show what appears at first 

 glance to be a flow structure, in fine white lines of feldspar. 



Under the microscope the rock is found to consist of green 

 hornblende and plagioclase, with accessory magnetite and apa- 

 tite. No quartz is present. The two essential minerals are 

 about equal in amount, though variations occur in which horn 



