560 Dr. J. W. Evans — Presidential Address. 



artificial mineral has been formed under the same conditions as the 

 natural product. 



For this purpose it is above all necessary that there should be in 

 the first place a systematic comparative study of these characters 

 and of the association in which they are found. The results thus 

 obtained should be of the greatest value in indicating the directions 

 along which experimental work would be most probably successful. 

 They should, of course, be supplemented by laboratory studies of 

 the relations of such subsidiary crystallographic characters to the 

 environment in the case of crystals which can be formed under normal 

 conditions of temperature and pressure, and therefore under the 

 immediate observation of the experimenter. Some work has, in 

 fact, already been done on the effects on these characters of the 

 presence of other substances in the same solution. 



In the study of the secondary alterations of metalliferous deposits, 

 especially those which consist of the enrichment of mineral veins by 

 the action of circulating solutions, either of atmospheric or intra- 

 telluric origin, the study of pseudomorphs gives, of course, valuable 

 assistance in determining the nature of the chemical and physical 

 changes that have taken place. 



A. successful solution of the problem of the exact conditions under 

 which deposits of economic importance are found would be of 

 incalculable value in facilitating their discovery and exploitation, 

 and would be the means of saving a vast amount of unnecessary 

 labour and expense. 



The problem of the structure and nature of the earth's interior, 

 inaccessible to us even by boring, would seem at first sight to be well- 

 nigh insoluble, except as far as we can deduce from the dips and 

 relations of the rocks at the surface their downward extension to 

 considerable depths. We can, however, gain important information 

 about the physical condition of the deeper portions from the reaction of 

 the earth to the external forces to which it is subjected, and still 

 more from a study of the " preliminary " earthquake tremors that 

 traverse it, the time occupied in their passage, and the difference in 

 intensity of those that follow different paths. These methods are, 

 however, not applicable to the earth's crust. Its physical characters 

 appear to be distinct from those of the interior, but very little is as 

 yet definitely known about them, except of course in the neighbour- 

 hood of the surface, and for this reason they are usually ignored in 

 calculating the paths of tremors traversing the earth. It seems to 

 be separated from the deeper portions of the earth by a surface of 

 discontinuity at which earthquake vibrations travelling upwards 

 towards the surface may be reflected. Calculations based on the 

 total time taken by these reflected waves to reach the surface after 

 a second passage through the earth's interior appear to indicate that 

 this surface of discontinuity, whatever its nature may be, is at a depth 

 of about 20 miles, though there can be little doubt that this depth 

 varies considerably from point to point. 



The main earthquake vibrations appear to follow the curvature of 

 the earth, and to be confined to its crust, instead of traversing the 

 interior, as is the case with the preliminary tremors. In these 



