784 DESCEIPTIVE GEOLOGY. 



compact and lithoidal. The latter pass into chalcedony, which covers the 

 weathered surface, and sometimes forms the mass of the rock in bands a 

 foot or more in thickness. 



At the head of a side-ravine, where, in a low saddle, the underlying 

 rocks have been denuded, are disclosed a most interesting series of rhyolitic 

 pearlites, chalcedonies, and tufas, which, from the occurrence of rounded 

 obsidian balls within the pearlite layers, have been designated the Ball 

 Rocks. The upper layers on either side of this saddle are composed of 

 the green rhyolite already mentioned, and layers of brown chalcedony, on 

 whose weathered surfaces are curious rounded excrescences, of concentric 

 structure, resembling the fungoid growths found on old tree-trunks. This 

 similarity is heightened by the color and interior banded structure of the 

 chalcedony, which resembles woody fibre. Within the chalcedony mass 

 are frequent druses, lined with white banded opaline agate, and containing 

 quartz crystals. Zirkel describes the microscopic structure of the chalce- 

 dony as consisting of concentric globules and botryoidal concretions in a 

 seemingly colorless substance, which, by polarized light, is seen to be an 

 aggregation of siliceous sphserulites. A section has been figured in Vol. 

 VI, Plate XII, fig. 2. 



On the saddle are exposed layers of pearlite, containing rounded ob- 

 sidian balls, from half an inch to an inch in diameter, associated with a 

 white pumiceous tufa, enclosing fragments, generally rounded, of the pear- 

 lite. The pearlite is of a blue-gray color, devoid of crystalline ingredients, 

 with a tendency to form layers from an inch upward in thickness. It has a 

 wavy appearance, and is entirely made up of sphaerulitic concretions. The 

 sphserulites have a concentric structure, and are formed of thin layers. 

 Under the microscope, these layers are seen not to be complete rings, but 

 to be grouped round the centre like the leaves of an onion, and the micro- 

 litic products of devitrification to be arranged in parallel wavy bands 

 through the mass, quite independent of the concentric structure, from which 

 Professor Zirkel concludes that this structure is merely a phenomenon of 

 contraction. The pumiceous tufa, which is found abundantly along the 

 slopes of the ridge, is a white ^porous mass, containing small fragmentary 

 crystals of quartz and sanidin, and enclosing larger fragments of the gray 



