SILICIOUS GEODES AND ETCHED CONGLOMERATE. 215 



surface by the weathering of the enclosing rock, and is confined largely 

 to their exposed upper sides. 



From its microscopic structure, the silica of which these geodes are 

 composed would appear to be but little more liable to solution than vein 

 quartz or quartzite. Minute quantities of amorphous silica may, how- 

 ever, be present and by their solution facilitate the removal of the crys- 

 talline grains by solution or otherwise ; hence less importance is attached 

 to the solution of these geodes than to the etching of conglomerate peb- 

 bles composed of various forms of silica which has been observed at sev- 

 eral rather widely separated points in the south. 



ETCHED CONGLOMERATE FROM NUTTALL, WEST VIRGINIA. 



The first case to be described is one observed by Mr Campbell at 

 Nuttall, West Virginia. The conglomerate in question, which belongs 

 to the coal measures, is composed of rather coarse quartz sand with 

 slightly yellowish cement, in which are embedded well worn pebbles of 

 white vein quartz. The latter vary in size up to three-quarters of an 

 inch in diameter and are somewhat irregularly distributed. Ordinarily 

 the pebbles, wholly unaltered, weather out by the chemical or mechanical 

 disintegration of the sandy matrix. In the case observed, however, where 

 the conglomerate receives the drip from an overhanging cliff, the pro- 

 jecting portions of the pebbles, as shown in plate 18, are deeply pitted, 

 evidently by solution. Mechanical wear in the formation of these cavities 

 is excluded by the form of the resulting surface. The latter is rough 

 and irregular, quite unlike the smooth portions of the pebbles where 

 still protected by the matrix. The outer portion of the pebbles is appar- 

 ently less easily affected by the solvent than the interior and forms a 

 sharp rim about the irregular cavities hollowed out within. In some 

 cases a third of the pebble has thus been removed. The surface of the 

 sandstone matrix in which the pebbles are embedded is also pitted, prob- 

 ably by the same process of solution, although such a surface might also 

 be produced by mechanical means in case the cement were less indurated 

 in some portions of the rock than in others. When the surface is closely 

 examined with a glass both processes are seen to have contributed to the 

 result. In some places the pitting has been produced by the solution of 

 the sand grains which are smoothed off evenly, and in others by the 

 removal of the cement, leaving the sand grains intact. 



ETCHED CONGLOMERATE PEBBLES FROM WHITE COUNTY, TENNESSEE. 



The second case observed is on Clifty creek, White county, Tennessee. 

 The conglomerate, in this case also a member of the coal measures, forms 

 the bottom of a small canyon and is covered by the creek at high water, 



