12 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



in effusive rocks. Spherulitic structure in particular is brilliantly 

 illustrated in the slides, and in the nature of the microlites of artificial 

 feldspar he has found nothing to suggest essential differences between 

 the experiments and natural processes the results of which constantly 

 come under the observation of lithologists. Professor Iddings has 

 also examined the refractive indices of the artificial feldspars and has 

 found them accordant with isomorphism. 



I can not conclude this review without a mention of the tireless 

 energy and watchfulness which Messrs. Day and Allen have exercised, 

 and of which I have been a daily witness, in a most laborious task 

 attended by so many difficulties that it sometimes seemed almost 

 hopeless. 



The vastness of the field open to geophysical research is partially 

 indicated in the preceding pages, and I have recently endeavored to 

 enumerate somewhat more fully the pressing problems of geophysics.* 

 The Government of the United States has of late years pursued the 

 enlightened policy of making yearly grants for chemical and physical 

 researches under the Geological Survey, but the appropriations are 

 inadequate for the more difficult and costly studies in this field. 

 This is not strange, for though geophysics has already proved techni- 

 cally valuable, so that mining engineers display a hearty interest in it, 

 and although it will assuredly lead to the solution of certain well- 

 defined economic problems of the first importance, its fundamental 

 researches are somewhat remote from industry, and large public 

 appropriations are hardly to be hoped for in the near future. 



The Trustees of the Carnegie Institution of Washington, recogniz- 

 ing these facts, have supplemented the Congressional appropriations 

 by grants to Dr. Day and to myself, and the work described in this 

 paper, though begun under the Survey, was completed at the expense 

 of the Carnegie Institution. In these circumstances the Director of 

 the Survey consented that it should be offered to the Institution for 

 publication with this due recognition of the cooperation of the Survey. 

 The Institution has accepted it as its first contribution to geophysics 

 and defrays the cost of publication. It is surely a matter of con- 

 gratulation that the Government and a private institution should 

 cooperate in the advancement of knowledge. Such an alliance 

 brightens the prospects of Science. 



* International Scientific Congress of St. Louis, 1904, printed in Science, Octo- 

 ber 28, 1904. 



