RELATIVE A(JE OE LAVAS. 251 



continuous bodies spread out over wide areas of country, especially along 

 the line of the Pinto fault, before the great bodies of the latter were forced 

 to the surface. Rhyolites occur breaking through the pumices, overflowing 

 and occasionally concealing them from view, except where the softer rock 

 is exposed by deep cuts along drainage channels. In some instances the 

 pumices lie superimposed upon denser rock, evidently of later age. It 

 seems most probable that throughout the duration of rhyolitic eruptions 

 conditions were at all times more or less favorable for the pouring out of 

 pumices and tuffs, and that outbursts of similar material began and closed 

 the rhyolite period. The conditions governing the physical characteristics 

 of the erupted material seem in a great measure to have been dependent 

 upon their relations to certain local centers of volcanic activity. 



Along the .Pinto fault, wherever the acidic lavas have piled up, 

 pumices occur as the prevailing rock, and the same holds true along the 

 lines of displacement bordering the elevated mountain masses. Normal 

 crystalline rhyolite, on the other hand, characterizes the Hoosac fault and 

 breaks out wherever these lavas penetrate into the interior of the mountains 

 along fissures and lines of least resistance. They frequently reach the 

 surface in small isolated bodies in the most distant and unlooked-for places- 

 The Rescue fault is an instance of rhyolite penetrating into the very 

 center of the mountains, and the pumices and tuffs on the south side of the 

 Silverado Mountains offer a fine example of the pouring out of the latter 

 along the outer edge of an uplifted orographic block. Following lines of 

 least resistance they connect the rhyolites of the Rescue with those of the 

 Pinto fault. 



When it comes to determining the geological relations of pyroxene- 

 andesite to hornblende-andesite and allied lavas, no direct superposition can 

 be found, nor are there any instances of dikes of one rock breaking through 

 an earlier body of the older rock. The main bodies of hornbleiide-andesite 

 and pyroxene-andesite are, as regards geological position and geographical 

 distribution, quite distinct. 



Absence of direct evidence as to the relative age of the two large 

 groups of andesite may be explained satisfactorily by the fact that only one 

 body of pyroxene-andesite occurs in the district and this one, although 



