_J.O CAI.IKORNIA ACADJilVIV OF SCIUNCJiS. 



in many cases these eruptions have also furnished large 

 amounts of effusive masses, but these subaerial flows have 

 long ago been eroded, leaving exposed the vents and fis- 

 sures through which they once poured forth. In continuing 

 its action the erosion has produced a peculiar type of moun- 

 tains, of which the Highwood and the Crazy Mountains 

 are good examples. I^oth are isolated volcanic districts 

 and consist of an intricate network of dikes and necks, in- 

 truded in sandstones and shales of Cretaceous or Laramie 

 aire. The erosion removes the softer sedimentary strata 

 much more rapidly than the eruptive rocks and the adjoin- 

 ing, slightly metamorphosed sandstones. The result is 

 an isolated group of mountains with extremely rugged and 

 serrated crests and ridges, rising abruptly several thousand 

 feet above the surrounding hilly or undulating country. 



A great deal of interest is attached to these volcanic 

 masses. The}^ expose to examination intra-telluric rocks, 

 consolidated under conditions greatly differing from 

 those to which the subaerial flows were subjected; having 

 at anv rate cooled very slowly and under very consider- 

 able pressure. The structure of the rocks is most gen- 

 erally holocrystalline-porphyritic. Glass basis is not fre- 

 quent, but may be noticed in a few instances. A certain 

 number of rocks have a holocrystalline-granular structure 

 in many cases connected with the porph3n-itic by transi- 

 tions. In a few cases the rocks are very coarse granular 

 and this occurs both in basic (Theralites, Wolff) and acid 

 members of the series. 



As to chemical composition these rocks appear to be 

 more varied than the series usually found in the Great 

 Basin; magmas rich in potassium are frequent, crystal- 

 lizing as trachytes; often they are very basic and contain 

 much sodium, resulting in the abundant separation of such 

 minerals as nepheline, sodalite and analcite. 



