GEOPHYSICAL LABORATORY. 99 



Mineral and Rock Densities. — Something over three years ago a method 

 was developed in the laboratory for the convenient measurement of the den- 

 sity of minerals and rocks at high temperatures. Comparisons of density 

 are commonly made at o° or at 25 °, but such determinations give little 

 information through which to compare densities, for example, at the tem- 

 perature of formation. In fact, throughout geological literature there is a 

 constantly recurring doubt as to whether, at the temperature of formation, a 

 solid mineral or rock is heavier or lighter than the molten mass out of which 

 it crystallized. With this new apparatus it is possible to make a continuous 

 series of measurements of density, upon small rock or mineral samples, from 

 comparatively low temperatures up to and beyond the melting-point, with 

 considerable accuracy. 



This is not a favorable opportunity to take up in detail the behavior of the 

 various minerals which have been studied in this apparatus, but a situation 

 has been developed through these experiments which is of considerable sig- 

 nificance to geologists. When a mineral or rock sample is heated, it almost 

 invariably begins to give off gas at about 900 , and may continue setting free 

 volatile material up to 1400 or 1500 . The disposition of the gas for the 

 purposes of this measurement offers no difficulty, but the interpretation of the 

 results becomes a somewhat complicated matter. The volume to which the 

 rock returns after the gas is set free is not identical with the initial volume, 

 nor is its expansion under these conditions an independent function of the 

 temperature. It follows that density determinations made at high tempera- 

 tures, but at ordinary atmospheric pressure, do not reproduce the conditions 

 which obtain during rock formation in nature, even though the tempera- 

 tures and the composition of the solid ingredients are the same. This may 

 serve as a further reminder that, in the general consideration of the physical 

 and chemical behavior of rocks and minerals in nature, regard must be had 

 tor the volatile ingredients which participated in their formation, whether or 

 not portions of these have escaped either at the time of formation or subse- 

 quently. Neither the physical and chemical properties of a rock nor its 

 relation to its neighbors remain the same after the volatile but chemically 

 very active ingredients have departed. 



Volcano Studies. — In pursuance of a plan which was approved last year 

 for the study of the physics and chemistry of active volcanoes, the Director 

 and one member of the laboratory staff spent three months of the past sum- 

 mer in studying the active crater Kilauea, on the Island of Hawaii. Of the 

 results little can be said until an opportunity has been given for a detailed 

 study of the products of volcanic activity which were collected and brought 

 to Washington for this purpose. In confirmation of the fact above men- 

 tioned, that the volatile ingredients have played a much more active part in 

 rock formation than has hitherto been attributed to them, it may be noted that 

 systematic observation has shown that the lava temperature within the active 

 basin is not constant, but varies within considerable limits from day to day, 



