PLATE XXX. 



Fig. a. 



(Sp. No. 33313. Without analyzer, x 38.) 



Photomicrograpli of a section of basalt from a pyroclastic sho'wing in ordinary light a distinctly 

 amygdaloidal character. The alteration of the hasalt has, however, reached such a stage that the 

 groundmass materials have for the most part heen completely altered, ivith the production of porphy- 

 ritic rhombohedra of calcite and plates of muscovite, which may he seen in abundance in the lower 

 right-hand side of the figure. (Described, pp. 129, 145.) 



Fig. B. 



{Sp. No. 33313. With analyzer, x 38.) 



Photomicrograph of the same section of basalt, showing in ordinary light a distinctly amygda- 

 loidal character when viewed between crossed nicols. The igneous texture is almost completely oblit- 

 erated by secondary products. The secondary calcite and muscovite stand out very sharply from 

 the very fine grained groundmass. (Described, p. 129.) 



290 



