VARIATIONS OF TEXTURE IN IGNEOUS ROCKS 589 



account of a liberal percentage of ferromagnesian minerals. It 

 has the appearance of a typical granular rock. Under the 

 microscope the structure is coarse. The largest grains are as 

 much as 8™" in diameter, and these grains, which have hypidio- 

 morphic or idiomorphic outlines, are closely intergrown. Filling 

 the spaces left by the intergrowth of these larger grains is a 

 somewhat scanty mesostasis, composed of grains averaging 

 about I™™ in diameter. The minerals of the larger and the 

 smaller grains are the same, consisting of feldspar, which is 

 greatly in excess, and quartz, green hornblende, and biotite. 

 The quartz is distinctly subordinate in amount to the feldspar. 

 The feldspars were tested by the Fouque method and gave, on 

 sections perpendicular to both the positive and the negative 

 bisectrices, the extinction angles for oligoclase. 



Hornblefide-biotite-quartz-diorite ( 123 N.) . — This does not differ 

 in the hand specimen to any noticeable extent from 117 N. 

 Under the microscope also the two are essentially alike, but with 

 a slight difference. In 123 N. the larger crystals are more 

 abundant, so that the structure at first sight appears to be coarse 

 allotriomorphic granular. On closer analysis the section is seen 

 to be made up of more or less idiomorphic crystals of feldspar, 

 green hornblende, and biotite, closely packed together, with the 

 spaces between filled with a scanty mesostasis of quartz, feldspar, 

 hornblende, and biotite, these grains being of all sizes, com- 

 mencing with the size of the idiomorphic grains just described. 

 In this section, as in 117 N., the quartz is a subordinate essen- 

 tial. The feldspar, tested twice by the Fouque method, gives 

 the angles for andesine-oligoclase. 



Hornblende-andesite (116 N.). — This is a porphyritic rock with 

 an abundant, greenish groundmass, and is entirely similar in 

 appearance to the hornblende-andesites of the Pinenut range, 

 just west of the butte. Under the microscope the groundmass 

 is found to be holocrystalline and microgranular, consisting 

 chiefly of allotriomorphic feldspar and hornblende. The pheno- 

 crysts reach a diameter of about 2.75°^'", and consist of feldspar 

 and green hornblende. The feldspars, when tested, gave twice 



