264 



GEOLOGY OF THE EUREKA DISTRICT. 



Lassen Peak iu northern California and are also regarded by him as of 

 primary origin.' 



Chemical Composition.— During the progress of the investigation upon the 

 erupted material, analyses were made of several of the more chai'aeteristic 

 rocks, which are presented here in tabular form arranged in the order of 

 their basicity. 



1. Coll. No. 163. — Rbyolite from Rescue Canyon. Analysis by R. W. Mahon. 1883. 



2. Coll. Ko. 121. — Rliyolitc from top of Pinto Pciik. Analysis by Dr. Edward Hart, of Lafayette 

 College. 1883. 



S. Coll. No. 175. — Rhyolite overlying dacite from northeast of South Hill. Analysis by R. W. 

 Mahon. 1883. 



4. Coll. No. 35.— Hornblendo-niica-andesite from hill northeast of Iloosae Mountain. Analysis 

 by R. W. Mahon. 1883. 



5. Coll. No. 1,9. — Dacite, small canyon northeast of South Hill. Analysis by R. W. Mahon. 

 1883. 



6. Coll. No. 71. — Andesitic-pearlite, south of Carbon Ridge. Analysis by W. H. Melville. 1890. 



7. Coll. No. 79. — Pyroxene-andesite, Richmond Mountain. Analysis by Dr. Thomas M. Drown, 

 Institute of Technology. 1883. 



S. Coll. No. ;?S4.— Basalt from saddle east of Basalt Peak. Analysis l)y Dr. Edward Hart. 1883. 

 9. Coll. No. 260. — Basalt, summit of Richmond Mountain. .Vniilysis by J. Edward Whit- 

 field. 1886. 



These nine analyses of carefully selected material represent the com- 

 position of the entire mass of extravasated lavas at Eureka and show a 

 range in their tenure of silica of over 25 per cent. Lavas from 1 to (J, 

 inclusive, belong to the feldspathic magma, and those from 7 to 9, 



'Am. .Jour. Sei., 3d .ser., 1887, vol. XXXIII, pp. 45-50. 



