ANDESITRS. ' 155 



half as much ag'aui alkali as the other rock. The difterence in the specific 

 gravities is also noteworthy. A coaiparison between these analyses and 

 those of some basaltic lavas will be made below. 



Andasites south of Clear Lake. — Mxtcnslvc arefts of andesitc occur to the south- 

 ward of Clear Lake. Mt. Cobb and Mt St Helena and, indeed, a great part 

 of the range of which the latter forms the culminating peak, known as tluf 

 Mayacmas Mountains, are andesitic. The andesites extend down almost 

 continuously to within a few miles of Vallejo, at the head of the IJay of 

 San Francisco. A considerable amount of reconnaissance work has been 

 done in this area for the purpose of studying the many quicksilver mines or 

 prospects which occur in the same region; but no attempt has been made 

 to map this andesite area or to work out the separate eruptions. Both dense 

 andesites of the earlier type and tlie asperites are represented. All the speci- 

 mens excepting one appear to be purely p3'roxenic, and no mica has been 

 observed. A single slide from near Napa City shows some small greenish- 

 brown 'hornblendes, accompanied by augite and hypersthene. As a rule 

 there is more hypersthene than augite in these rocks, but this relation is 

 sometimes reversed. The pyroxene never enters in considerable quantities 

 into the composition of the gronndmass. The feldspars, both porphyritic 

 and microiitic, in the Mayacmas Range andesites seem to be mainly labra- 

 dorite, a fact of special interest in view of the abnormal acidity of some of 

 them. The gronndmass of most of the slides shows feldspar microlites and 

 magnetite embedded in glass, which is sometimes partially devitrified and 

 sometimes full of trichites. The glass often shows a banded or rhyolitic 

 structure. 



To the south of the estuary of the Sacramento River volcanic rocks 

 are much less abundant than to the north, but they are not entirely want- 

 ing. Wliile hornblende is unusually rare in the northern andesites, aspe- 

 i-ite, carrying both hornblende and mica, occurs near Mt. Diablo. Professor 

 Whitney' mentions a belt of "trachyte" some fifteen miles northeast of 

 Tres Pinos. It may be considered certain that asperite is meant. My 

 party has not visited this locality, but we collected pebbles from compar- 

 atively recent gravel beds and from the present streams between Tres 



' Geol. Survey California, Geology, vol. 1, p. 47. 



