VOLCANIC! BOOKS OF SEPULCHRE MOUNTAIN. 121 



magma at different times. The order in which the different minerals began 

 to crystallize in that portion of the magma, which formed diorite appears 

 to have been as follows: Magnetite, hypersthene, augite, labradorite, horn- 

 blende, biotite, oligoclase, orthoclase, quartz. The feldspathic minerals 

 started to crystallize before any of the ferromagnesian minerals had com- 

 menced; and the last of the series undoubtedly crystallized after all of the 

 ferromagnesian minerals had been completed. So far as the siliceousness of 

 the minerals is concerned, the series of ferromagnesian silicates and that 

 of minerals free from iron vary in opposite directions. In the former the 

 range is from highest silica to lowest; from the metasilicate, hypersthene, to 

 the orthosilicate, biotite. In the second it is from the least siliceous, labra- 

 dorite, to the most siliceous, orthoclase, or to free silica, quartz. 



THE VOLCANIC ROCKS OF SEPULCHRE MOUNTAIN. 



The igneous rocks of Sepulchre Mountain are partly extrusive, partly 

 intrusive. By far the greater mass consists of subaerial breccias and tuffs, 

 with a small amount of massive lava flows The intrusive rocks form dikes 

 and larger bodies traversing these breccias. The breccias and flows are 

 andesites of various kinds. The intrusive bodies are andesites and dacite, 

 grading into porphyry-like modifications in places. The tuff-breccia is 

 separable into an older and a newer, or into a lower and an upper, breccia. 



THE LOWER BRECCIA. 



The lower breccia is about 500 feet thick, and consists mostly of frag- 

 ments of hornblende-mica-andesite, and is generally light colored. It carries 

 a large amount of fragments of crystalline schists, which do not occur in the 

 overlying upper breccia. It is probable that the lower breccia was ejected 

 from some neighboring center of eruption located in an area of Archean 

 rocks. Such a center occurs a few miles north, at the west base of Sheep 

 Mountain. The lower breccia passes into fine tuff in places, and at the 

 extreme end of the northwestern spur of the mountain it is distinctly bedded, 

 with layers containing bowlders of a rhyolite-porphyiy, which has not been 

 found in place in this region. In places the upper part of the breccia is 

 green and partly decomposed, as though weathered before the upper breccia 

 had been thrown upon it, In the northwestern spur of the mountain the 

 upper breccia is seen to rest upon an uneven surface of the lighter-colored 



