556 STUDIES FOR STUDENTS 



EXPLANATION OF PLATE VII 



Fig. i. — Section No. 4721. (x 12.) 1 Red sandstone from LaValle. 

 This is an excellent illustration of a sandstone in which the grains 

 were originally uniformly well rounded, and later enlarged and 

 cemented with silica. The enlargements are nicely shown in many 

 places in the section. The brown rims of iron oxide which separate 

 the original grains from the secondary quartz are very distinct in the 

 figure. 



Fig. 2. — Section No. 4720. (X 12.) Brown sandstone from 

 Argyle. This section illustrates a rock in which the grains are well- 

 rounded and cemented with iron oxide, but in which the individuals 

 have not been generally enlarged with secondary quartz. On account 

 of this the stone is considerably weaker than the one from LaValle. 



1 Magnified twelve diameters. 



