PLATE XII 



MAGNIFIED SECTIONS SHOWING ALTERATION IN BASALT 



Fig. 1. Breccia of an original glass, whose interstices have been filled with 

 ferruginous clay from the lake bottom. Recrystallization of the 

 glass has resulted chiefly in albite and chlorite. X 24. Slide 12. 



Fig. 2. Incipient stage of zeolitic alteration of glass, showing effect of chill 

 cracks in directing percolation of solutions. X 24. Slide 30. 



Fig. 3. Alteration effects in brecciated glass. X 24. Slide 61. 



Fig. 4. Prominent banding in secondary products, due to cracks in the original 

 glass. The darker portions are chiefly chlorite, the lighter analcite. 

 X 24. Slide 60. 



Fig. 5. Geometrical patterns in garnet, interpreted as due to resorption of 

 olivine. Gr = garnet, Am =: amphibole, Pr = prehnite. X 32. Slide 

 115. 



Fig. 6. Incipient alteration of glass, showing accentuation of physical and 

 chemical differences by a slight degree of alteration, x 35. Slide 54. 



