596 
GEOPHYSICS: R. B. SOSMAN 
Proc. N. a. S. 
weathering, and soils" about fifteen years ago, which summarized our 
knowledge up to that date. It involves physical factors such as the dis- 
integrating action of periodically fluctuating temperatures, together with 
the study of relative rates of reaction within and in the presence of dilute 
water solutions, with the complications due to colloidal phenomena and 
reaction in capillary spaces — a difficult field and one worthy of well- 
planned effort. 
Rock disintegration, corrasion, and transportation, included in the 
general process of erosion, bring in physical and mechanical problems such 
as the rounding of fragments by attrition; their sorting by air and water 
movements; their distribution and redistribution through the agency of 
winds, currents, and waves; the production of particular structures such 
as ripple marks ; the movement of unconsolidated sediments as in sand 
dunes, soil creep, and solifluxion; and similar questions. Reference need 
only be made here to summaries of these problems by Vaughan and by 
Merwin before the Geological Society of America in December, 1919. 
Experimental physics will be found deeply involved in all such problems. 
Colloidal chemistry will also enter, as, for instance, in the question of the 
precipitation and re-solution of fine suspensions; likewise biological 
chemistry, in the precipitation and consolidation of calcium carbonate, 
ferric oxide, and other products associated with organisms. 
Chemical questions allied to those of weathering will enter into the 
problems of the consolidation and alteration of sedimentary rocks, in- 
cluding cementation and recrystallization, the formation of low-tempera- 
ture veins, silicification, and the growth of concretions. The elastic con- 
stants of porous aggregates offer an example of the physical data that are 
likely to be needed in this same connection. 
Pyroclastic Rocks. — The question of the origin of these rocks brings 
up the problem of explosive volcanism. In its larger aspects this belongs 
properly to the Section of Volcanology, but the products of this type of 
volcanic activity are very wide-spread, and the particular physical and 
chemical questions arising from the state of subdivision and modes of 
distribution and alteration of the products deserve special mention in 
this rather rough grouping of geophysical-chemical problems. Examples 
are: the molecular mechanism of the explosion, the peculiar properties 
of flowing dust-clouds, the physics of the projection of volcanic bombs, 
the sorting of products by air currents and the weathering of "ash" to 
form colloidal products. 
The Metamorphic Rocks. — The point where a rock ceases to be "igneous" 
or "sedimentary" and becomes "metamorphic" is not now exactly de- 
fined, though subject to exact definition, but a distinct set of physical and 
chemical questions undoubtedly enters into the problems of dynamic 
and thermal metamorphism. The effects of non-uniform pressure in 
causing the flowing of crystalline substances and aggregates and their 
