214 C. W. HAYES — SOLUTION OF SILICA. 



talline rocks, as diabase and granite, from the fresh condition to that of 

 soil, they suffer a larger ^absolute loss of silica than of any other con- 

 stituent, the loss, however, being relatively greatest in those rocks which 

 contain least free quartz. 



Further, all river waters are known to contain more or less silica in 

 solution, amounting in some cases, as the Saint Lawrence and Ottawa, 

 to nearly a third of the total inorganic residue, but its presence is ordi- 

 narily attributed to the decomposition of silicates in crystalline rocks. 

 While these facts are generally recognized, free silica is commonly re- 

 garded as the mineral least liable to be affected by solvents under 

 ordinary atmospheric conditions, and a quartz pebble is expected to 

 withstand all ordinary vicissitudes except the slow loss by mechanical 

 attrition. It is doubtless true that, considered relatively to other rock- 

 forming minerals, quartz is one of the most obdurate; but some recent 

 observations show that under certain conditions now existing at the 

 earth's surface free silica, quartz as well as its other forms, is by no 

 means proof against chemical as well as mechanical agents of erosion. 



Observed Cases of Solution of Silica under atmospheric Conditions. 

 silicious geodes from spurrier, tennessee. 



The first to be described is that observed in the solution of silica in 

 geodes collected by Messrs Campbell and Taff at Spurrier, Overton 

 county, Tennessee. These silicious concretions occur rather abundantly 

 in certain layers of the Carboniferous limestone, accumulating at the 

 surface on the removal of the latter. They are of all sizes up to a foot 

 or more in diameter, and generally have a rugose surface, somewhat re- 

 sembling the bark of a tree. The outer portions are yellowish or white, 

 and have an opaque, stony appearance, while the interior is colorless 

 and translucent, resembling extremely fine grained quartzite or slightly 

 granular vein quartz. Under the microscope this is seen to be made up 

 largely of spherulitic aggregates of quartz, the elongated grains of quartz 

 having a radial arrangement. There are also considerable areas of rather 

 coarse, granular quartz, often with terminated crystals, but no cryptocrys- 

 talline or amorphous silica could be detected. Many of these geodes, as 

 shown in plate 17, figure 2. are deeply etched upon one or both sides, not 

 only the outer opaque shell being removed, but portions of the translu- 

 cent interior. A part of this etching was probably accomplished by the 

 solvent which removed the limestone, as one specimen at least was found 

 embedded in the residual clay between two layers of limestone, evidently 

 not far removed from its original position in the rock. In most cases, 

 however, the solution was evidently effected after the geodes reached the 



