PLATE XXIX. 



Fi&. A. 



(Fp. No. 32582. Witliout analyzer, x 38.) 



Photomicrograpli sho^viiig the normal igneous texture of a basalt. The area formerly occupied 

 hy the feldspar substance is now occupied by a granular aggregate of various minerals. (Described, 

 p. 127.) 



Fig. B. 



(Sp. No. 32582. With analyzer, x 38.) 



The same section of basalt when viewed between crossed nicols. The only indication of an 

 igneous texture is shown by the amygdnlcs present. The igneous texture of the rock is completely 

 concealed as soon as the aggregates occupying the feldspar areas break up into their constituent grains. 

 (Described, p. 127.) 



288 



