582 wherry: clay from volcanic dust 



can continent during much of Cretaceous time, the dust would 

 naturally become spread more or less evenly and widely over the 

 bottom. In settling it would, furthermore, undergo at least 

 partial sorting into heavier and lighter, or coarser and finer, 

 particles, and develop some degree of stratification. Grains of 

 feldspar, quartz, hornblende, mica, and other crystalline minerals 

 would settle more rapidly than those of volcanic glass, full of 

 gas-bubble cavities. The lower part of any stratum would thus 

 naturally contain the bulk of the crystalline minerals present in 

 the dust, the upper part most of the glass. 



The crystalline minerals, not having been long exposed to 

 atmospheric weathering before deposition, should appear in the 

 sediment in essentially fresh condition. The volcanic glass, on 

 the other hand, being porous and charged with gases, — includ- 

 ing, no doubt, hydrochloric acid, sulfur dioxide, and other 

 chemically active gases from the volcanic emanations, — would 

 be likely to undergo "auto-metamorphism," in the sense in 

 which this term has recently been used by H. C. Sargent. 8 The 

 product would naturally be expected to have peculiar properties. 

 When it is considered that rain water containing traces of carbon 

 dioxide, acting on crystalline feldspars at the earth's surface, 

 often produces amorphous halloysite ("kaolinite"), the proba- 

 bility of the development of an amorphous, colloidal clay-ma- 

 terial from the originally amorphous volcanic glass in this case 

 is evident. In the former instance most of the alkalies and alka- 

 line earths and part of the silica are dissolved and carried away 

 by the rain water; in the latter, water circulation and diffusion 

 would necessarily be so hampered that little removal of constit- 

 uents could be expected, except perhaps the most soluble 

 ones, the alkalies. The clay should therefore differ from the 

 original glass chiefly by the presence of more water, and less 

 alkalies. How this is borne out by the composition actually 

 observed is shown in the analyses given in table 1. 



The clay here described is hardly to be regarded as a definite 

 mineral. It more probably consists of mixed gels of alumina 

 and silica, with adsorbed alkalies and water. Resemblances in 



8 Paper given before the Geological Society of London; abstract in Nature, 

 99: 59. 1917. 



