232 



I. H. OGILVIE 



hornblende and of quartz may also be seen. The ground mass 

 consists of fine grains of plagioclase with augite and magnetite. 



Analyses i and 2 were from specimens taken from two sheets, 

 respectively on the west and the southwest of the group. It is note- 

 worthy that all of the FeO is in magnetite and ilmenite and that the 

 normative diopside and hypersthene are of a pure magnesian variety. 

 The second analysis is close to a division line in both order and rang, 

 so this type is properly a Laurvikose-Yellowstonose-Lassenose. 

 Norm and mode agree fairly well. 



Yellowstonose. I. 4. 3. 4 (Dacite) 

 This type is in all megascopic and microscopic respects similar 

 to the Lassenose. It is a porphyritic dacite, and occurs in sheets. 

 The sole important difference is that potash, and hence the sum of 

 the alkalies, is lower in Yellowstonose, thus placing the rock in rang 3, 

 in spite of the fact that lime and soda are present in approximately 

 equal amounts in both types. The second analysis (IV) approaches 

 rang 2 and might be termed a Lassenose- Yellowstonose. 



The sheets from which specimens for analysis were taken are 

 respectively on the northeast and southeast sides of the mountains. 



TABLE III 



Chemical Composition of Yellowstonose 



