WRIGHT: MICROMETER OCULAR 



61 



show in convergent polarized light either both optic axes or only- 

 one in the field of view. 2 



The primary purpose in constructing this coordinate microm- 

 eter eyepiece has been to produce an accurate, simple measuring 

 device which covers the entire field of view and with which the 



f - .RAMSDEN 

 OCULAR 



-OCULAR 

 SUPPORT 



.-COORDINATE 

 SCALE 



Fig. 1. Section of micrometer ocular 



optic axial angles of minerals in the thin section can be measured 

 satisfactorily and geometric analyses of rock thin sections accom- 

 plished by either one of the two available standard methods. 

 This eyepiece may well replace the single line micrometer eyepiece 

 which is ordinarily furnished with petrographic microscopes. 



PETROLOGY. — The lavas of Hawaii and their relations. 

 Whitman Cross G3ological Survey. 



Introduction. — The writer has undertaken a study of the lavas 

 of the Hawaiian Islands with the dual object of contributing to 

 the petrography of this interesting province and of establishing 

 a basis for testing certain current broad generalizations concern- 

 ing the genetic relations of igneous rocks. The generalizations 

 in question are: (1) that the so-called alkali and subalkali groups of 



2 Compare F. E. Wright, Amer. Jour, of Sci. (4), 24: 341. 1907; 29: 423. 1910; 

 31:204. 1911. 



