MICROSCOPIC STRUCTURE OF SANDSTONE DIKE 



359 



continuity with the original grain, changing it from a rounded to an 

 angular outline and filling all the interstices. Figures 7 and 8 show 

 such grains of sand, with the secondary quartz unshaded. Viewed in 

 ordinar}' light, the grain is seen to be outlined by a dark line, and fur- 

 thermore differentiated from the new quartz by a cloudy appearance, 

 due to numerous inclusions, from which the new quartz is free. This 

 latter is scareely visible in ordinary light, but with crossed nicols shows 

 a continuation of the colors of the original grain beyond the dark line, 

 which is still visible. The whole extinguishes together. 



Figures 7 and 8. — Sand Grains from Dike Enveloped in Secondary Quartz (greatly enlarged). 



As seen in section, the enlarged grains have very irregular outlines, 

 with frequent reentrants, occupied by the new quartz of an adjacent 

 sand grain. In the immediate vicinity of an included limestone frag- 

 ment there is a strong intermingling of the quartz grains with the lime- 

 stone. The quartz grains retain their original outline, without secondary 

 enlargement, the limestone forming the cement between them. This 

 limestone cement has the same appearance as the solid limestone, being 

 fine grained, granular, and of a grayish color. 



Pure calcite is rare, there being apparently little recrystaliization from 

 solution, except near the borders of the dike. While the outline of the 

 sand grains is usually sharp, it is not infrequently an irregular one, the 

 limestone matrix appearing as if pressed into the sand grain, thus giv- 

 ing it an irregular border. Individual grains are not infrequently in- 

 cluded in a mass of limestone, apparently not differing from the main 

 mass of the limestone. These grains retain their original outline, not 



