104 UNIVERSITY OF COLORADO STUDIES 



ratio of ferromagnesian minerals to light-colored constituents. Pink 

 and purplish colors sometimes present in the feldspars may modify 

 the tones, but probably nowhere is the unaltered rock red. Weathered 

 surfaces may sometimes be stained red or brown by ferric oxide, and 

 locally weathering may extend to considerable depths. The first 

 product of disintegration is a mixture of angular fragments of feldspar 

 and quartz, gray to brownish-gray in color. Further decay, where the 

 rock is protected from rapid erosion, results in a clay-like product 

 brownish-yellow in color. 



The megascopic minerals are feldspar, quartz, biotite and horn- 

 blende named in the order of importance. The feldspar is commonly 

 white, but pink and purplish crystals are not rare. Individuals vary in 

 size from grains which can barely be distinguished by the aid of a lens 

 to irregular masses 2 cm. in diameter, and in the porphyritic variety, 

 to crystals 5 cm. long. Quartz, somewhat less abundant than feldspar, 

 occurs in grains usually less than 1 cm. in diameter. The ferromag- 

 nesian minerals are variable in quantity; in places they almost disap- 

 pear, yet frequently are so plentiful as to give the rock a very dark 

 color. Biotite is nearly always present and generally to the exclusion 

 of hornblende, though locally the granite contains biotite and horn- 

 blende in nearly equal amount, and sometimes grades into a granodiorite 

 with a large proportion of hornblende and little or no biotite. 



Under the microscope microcline is seen to be the most important 

 feldspar, sometimes fully equal to the combined amounts of all other 

 constituents. It invariably shows polysynthetic twinning after the 

 albite and the pericline laws. Crystal outline is completely lacking, 

 the mineral having crystallized contemporaneously with quartz and 

 orthoclase. Micropegmatitic intergrowths of quartz and microcline are 

 rarely present; sometimes the two minerals are intergrown in such a 

 manner that tongues of one mineral penetrate the other. The micro- 

 cline is usually quite fresh, though sometimes it is turbid from kaoliniza- 

 tion and less often shows alteration to sericite. The phenocrysts of 

 the porphyritic facies are, in part at least, microcline. 



Orthoclase is generally much less abundant than microcline; while 

 it may rarely equal or even exceed microcline in amount it is almost 



