574 DESCRIPTIVE GEOLOGY. 



r 



Its chemical constituents, as determined by Mr. R. W. Woodward, are: 



Silica 58.54 58.24 



Alumina 16.68 16.85 



Ferrous oxide 5.62 5.59 



Lime 6.00 5.92 



Magnesia 5.22 5.33 



' ' Soda 2.76 2.78 



Potassa... 2.50 2.50 



Water.--. ...- 2.15 2.23 



99.47 99.44 



This diorite forms bold hills to the east of Agate Pass Road, through 

 which are eroded gentle rounded canons. There is singularly little dioritic 

 ddbris, almost all the disintegrating material being readily ground up into 

 fine sand, so that few boulders of diorite are found on the surface. 



Directly to tlie east of the diorite is a very great thickness of siliceous 

 and gritty beds, striking a little east of north, and dipping to the west, which 

 in their composition and texture present a striking resemblance to the 

 peculiar quartzitic sandstones already described from the Fountain Head 

 Hills and Tucubits Mountains, though, in general, possessing a more 

 massive habit. They consist chiefly of yellow and saffron-colored sand- 

 rocks, containing a considerable proportion of broken feldspar crystals, with 

 here and there sheets of siliceous cherty pebbles, black, red, and white, both 

 angular and round. They contain also a small percentage of carbonate of 

 lime, which is due to the presence of microscopic crystals of calcite. The 

 quartz-grains contain fluid-cavities with moving bubbles, and the feldspars 

 are usually quite earthy and decomposed. A peculiar feature of this rock 

 are the vugs lined with crystals of quartz and carbonate of lime. All the 

 base of the rock is a fine clastic material, and a thin translucent chalcedony 

 fills the interstices. Among the included pebbles and conglomerates are 

 many of a small, brown hornstone or chert, which often occurs in the sili- 

 ceous beds of the conglomerate, but have nowhere been found among the older 

 rocks and clays. The angular condition of many of tlie included frag- 

 ments is not surprising in view of the granite mass, directly to the north of 



