216 C. W. HAYES— SOLUTION OF SILICA. 



but is uncovered throughout the greater part of the year. The matrix is 

 a coarse, white sandstone which weathers yellow by the oxidation of the 

 slightly ferruginous cement. Embedded in this are rather abundant 

 pebbles varying in size, the largest being two inches in diameter, and 

 composed chiefly of white vein quartz and fine grained vitreous quartzite, 

 together with a few of chert. As shown in plate 17, figure 1, the pro- 

 jecting portions of these pebbles have been in part removed, although 

 they still project somewhat above the enclosing matrix. As in case of 

 the Nuttall conglomerate, the exterior portions of the pebbles are less 

 easily affected than the interiors, and when the pebble has been a third 

 or half removed the outer shell forms a rim, within which is a depression 

 with a slight elevation in the center. The chert pebbles show less evi- 

 dence of corrosion than those composed of quartz. Their upper surfaces 

 are somewhat worn down and even slightly hollowed, but this might 

 have been produced by mechanical means, which is not the case with 

 the quartz. One of the chert pebbles is cut by a thin vein of white quartz 

 evidently in place before the pebble was formed. The quartz in the vein 

 has been removed somewhat below the surface of the enclosing chert, 

 having apparently been more readily attacked by the solvent than the 

 latter. However surprising this conclusion may be, it is supported by a 

 comparison of numerous pebbles which appear to have been under es- 

 sentially the same conditions. Those composed of white vein quartz 

 were found generally to be most deeply etched, those composed of chert 

 were least affected, while the quartzite pebbles appeared to occupy an 

 intermediate position between the other two kinds. 



ETCHED CONGLOMERATE FROM STARRS MOUNTAIN, TENNESSEE. 



The third case observed is in a conglomerate of lower Cambrian age 

 from Starrs mountain, Tennessee. The matrix is a coarse, feldspathic 

 sandstone containing layers of well rounded pebbles, mostly quartz, with 

 a few of some feldspar much altered. The former are between half an 

 inch and an inch in diameter and the latter are somewhat larger. The 

 projecting portions of the quartz pebbles on one side of the block shown 

 in plate 19, are almost entirely removed and, as in the other cases, evi- 

 dently by solution. 



The accompanying diagram, figure 1, shows the form assumed by the 

 majority of these pebbles as well as those at Clifty creek described 

 above. Ordinarity a distinct rim projects above the matrix in which 

 the pebbles are embedded. Within this is a depression, while a slight 

 elevation occupies the center. The rim is sometimes absent, and the 

 surface of the matrix is flush with that of the pebble. The etching is 

 not confined to the larger pebbles, although it is there most noticeable, 



