RELATIVl': ACK OF LAVAS. 251 



continuous bodies spread out over wide aix-as of country, especially along- 

 the line of the Pinto fault, Ix'fore tlie great bodies of the latter were forced 

 to the smface. Klixoliti's occur l)reaking- througli the pumices, overfowing- 

 and occasionalK' (•(mccaliug them from view, exccjit wliere the softer rock 

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

 pumices lie su})eriiiip(i.scd upon denser rock, evidently of later age. It 

 seems most probable that tlirougliout tlie duration of rhyolitic erujjtious 

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

 pumices and tuffs, and that outbiu'sts of similar material began and closed 

 the rhyolite period. Tlie conditions governing the physical characteristics 

 of the erupted material seem in a great measure to have been dej^'udcnt 

 upon their relations to certain local centers of ^'olcanic activity. 



xA.long the Pinto fault, wherever the acidic lavas liave })iled up, 

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

 lines of displacement bordering- tlie elevated mountain masses. Normal 

 crystalline rhvolite, on the other hand, characterizes tlu' Iloosac fault and 

 l)reaksout wherever tliese lavas penetrate into the interior of the juountains 

 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 piimices and tuffs on the south side of the 

 Silverado Mountains offer a fine exnmple of the pouring- out of the latter 

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

 least resistance the)' connect tlie iliyolites of the Rescue \\ith those of the 

 Pinto fault. 



When it comes to determining- the geological relations of p}-i-oxene- 

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

 l)e found, nor are there :\nv instances of dikes of one rock breaking tlii-ougli 

 ail earlier bo(h- of the older rock. The main bodies of horiddende-andesite 

 and ])vroxene-andesite are, as regards geological position and geographical 

 distril)Ution, ([uite distinct. 



Absence of direct e^ddence as to the relative age of the two large 

 groups of andesite may l)e explained satisfactorily by the fact that onlv one 

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



