320 ABSTRACTS 



that described above. They vary from three inches to a foot in diam- 

 eter and beneath the rugose opaque surface are colorless and trans- 

 lucent, resembling in texture extremely fine-grained quartzite or 

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

 made up of spherulitic aggregates of quartz grains with much coarse 

 granular quartz but no cryptocrystalline or amorphous silica could be 

 detected. Many of these geodes are deeply etched upon one or both 

 sides, not only the opaque shell being removed but also portions of 

 the translucent interior. From its microscopic character, the silica of 

 which they are composed would appear to be but little more liable to 

 solution than vein quartz or quartzite. Minute quantities of amorphous 

 silica may, however, be present and by their solution facilitate the 

 removal of the crystalline grains by solution or otherwise. Hence less 

 importance is attached to the solution of these geodes than to the 

 etching of the conglomerate pebbles. 



The wide separation of the localities at which these cases of the 

 undoubted solution of silica occur, renders it improbable that they are 

 due to some peculiar and exceptional conditions, such as the presence 

 of thermal, alkaline waters. It seems rather that they must be attrib- 

 uted to the action of widespread agencies working in these cases under 

 more than ordinarily favorable conditions so that the effect is excep- 

 tionally striking. All the cases occur in a heavily forested region 

 where there is a thick layer of humus and doubtless an abundant sup- 

 ply of the humus acids. Further, the region is one in which forest 

 fires are frequent and considerable potassium carbonate must be sup- 

 plied to surface waters by leaching of the resulting ashes. The con- 

 ditions, therefore would appear to be favorable for a chemical process 

 somewhat as follows : By the oxidation of the vegetable tissues in the 

 process of decay, the humus acids are formed, chiefly humic and 

 crenic. These absorb varying quantities of free nitrogen from the air 

 forming the azo-huraic acids, which, in turn, combine with free silica. 

 The resulting acids combine with alkaline carbonates, particularly 

 potassium carbonate, to form easily soluble salts. In most cases the 

 etched surfaces of the pebbles described above support a more or less 

 abundant growth of cryptogamic vegetation which might facilitate the 

 solution in two ways: first by supplying humus acids directly from its 

 own decay, and, second, by absorbing solutions of those acids from 

 other sources and keeping the rock surfaces moist so that their solvent 

 action might be practically continuous. 



