230 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



special mention in this rather rough grouping of geophysical-chemi- 

 cal problems. Examples are : the molecular mechanism of the explo- 

 sion, 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 defined, though subject to exact definition, but a distinct set 

 of physical and chemical questions undoubtedly enters into the prob- 

 lems of dynamic and thermal metamorphism. The effects of non- 

 uniform pressure in causing the flowing of crystalline substances and 

 aggregates and their solution and redeposition, as well as the forces 

 exerted in their recrystallization, are of particular interest. The 

 structures resulting from movement in silicate rocks under differ- 

 ential pressure are in need of quantitative study. Transfers of 

 mechanical energy and its conversion into heat are also involved. 

 Consolidation and recrystallization as a direct result of uniform 

 pressure are likewise to be considered. 



Mention should be made in this connection of the physics and 

 chemistry involved in faulting as well as in jointing and minor move- 

 ments of the solid rocks. These phenomena have often been treated 

 under the principles of elastic theory as applied to homogeneous 

 bodies, yet there can be no question that the elastic properties and 

 conditions of rupture of aggregates must differ in many essential par- 

 ticulars from those of homogeneous bodies. Here is a considerable 

 field for experimentation. 



Bodies of water and the " chemical sediments ". — The chemistry of 

 the deposition of salts from sea water has already been made the 

 subject of special research, and van't Hoff's results in this field are 

 already familiar. The deposition of calcium carbonate awaits a simi- 

 lar thorough study. Allied questions are the formation of dolomite, 

 the deposition of various salts from inclosed bodies of water, the 

 deposition of phosphate rocks, the precipitation of colloidal suspen- 

 sions of clay and other substances, and the origin of the great deposits 

 of sedimentary iron ore. 



Problems of fresh-water bodies and streams include the chemistry 

 of bog-iron ores, and the amounts of insoluble and colloidal solids and 

 of soluble salts carried by streams. Problems relating to under- 

 ground waters may also be included here, such as : the dissolving and 

 recrystallizing activity of underground waters ; connate waters and 

 their possible chemical changes; movements of underground waters 

 and their relation to the alteration and concentration of gas and oil ; 

 principles of hydraulics governing the flow of wells and springs ; and 

 the characters which will serve to differentiate between vadose and 

 juvenile waters. 



