OCCURRENCE A¥D SUCCESSION OF ROCKS. 207 



tions, there appears no reason to suppose that vast quantities of heated 

 fluids may not be driven to the surface without an accompaniment of lava. 

 The solfjitaric action and the fault are certainly contemporaneous, and may 

 together form the entire volcanic manifestation of the period in which they 

 occurred. If the}^ were independent of the eruption of younger horn- 

 blende-andesite, they must have been subsequent to it; and I beheve it 

 most probable tliat such was the case. Of their time relations to the basalt 

 eruption there is no means of judging. 



Collections. — Tlic disputcd charactcr of a number of the rocks of the 

 District made very full collections essential to the substantiation of the 

 views maintained in this report. A cabinet series of 200 specimens was 

 collected in triplicate, one set being designed for the lithological collection 

 of the National Museum, a second for the geographical collection of the 

 same institution, and a third for the San Francisco office of the Geological 

 Survey. By order of the Director, the size of these specimens is 4 inches 

 by 5 inches, their thickness being from an inch to an inch and a half. 

 Though these specimens, selected with a view to representing -the District 

 as well as possible, amply suffice for the ordinary purposes of study, so small 

 a number was not found sufficient to justify the geological map and sections. 

 A working collection without duplicates was therefore also gathered. The 

 size adopted was only If by 2^ inches, in order to lessen the labor of 

 gathering them and to facilitate their use in the office. This collection con- 

 tains over 2,000 numbers. Slides were ground whenever they seemed likely 

 to aff"ord desirable information, and the total number cut was about .oOO. 



The locality of every specimen was recorded at the time of collection 

 on a map of the surface or of the mines as the case might be. The mine 

 maps employed were on a large scale, and the localities are usually accurate 

 to three or four feet. The surface map being on a comparatively small scale 

 the positions are less precise, but are recorded as accurately as practicable. 

 On the sections of the Lode, shown in the Atlas, the points from which speci - 

 mens were collected are marked by crosses, while each locality from wliich 

 there is a slide is indicated by a large black dot. On the surface map ;i. 

 red cross shows the localities microscopically determined, a single cross 



