GEOPHYSICAL-CHEMICAL PROBLEMS SOSMAN. 229 



tion of the permeability of crystalline substances to volatile com- 

 ponents may perhaps arise. Studies of the inversion of polymorphic 

 forms, properties of liquid inclusions, and the effects of increased 

 temperature on neighboring rocks must be made in order to obtain 

 data on the original temperature of the magmas. 



Sedimentary rocks. — No sooner does the solidified igneous rock 

 find itself at or near the earth's surface than it becomes the object 

 of the group of processes known as " weathering." This is a subject 

 that has been nearly at a standstill since the publication of Merrill's 

 " Rocks, rock weathering, and soils " about 15 years ago, which sum- 

 marized our knowledge up to that date. It involves physical factors 

 such as the disintegrating action of periodically fluctuating tem- 

 peratures, 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 particu- 

 lar structures, such as ripple marks; the movement of unconsoli- 

 dated 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-tem- 

 perature veins, silicification, and the growth of concretions. The 

 elastic constants 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 be- 

 longs properly to the Section of Volcanology, 1 but the products of 

 this type of volcanic activity are very widespread, and the particular 

 physical and chemical questions arising from the state of subdivision 

 and modes of distribution and alteration of the products deserve 



1 Of the American Geophysical Union. 

 101257—22 16 



