246 J. F. KEMP AFTER-EFFECTS OF IGNEOUS INTRUSION 



the adjective aqueo-igneous. That igneous vapors or mineralizers were 

 of moment in their formation has been long recognized. Except as the 

 home of cassiterite, the common ore of tin, however, typical pegmatites 

 were not often productive of ores in earlier years and were not given much 

 attention by mining geologists. With the increasing use of tungsten, 

 molybdenum, and the rare earths, the pegmatite veins or dikes have been 

 of growing significance in their bearing on our interpretations of vein 

 phenomena. 



If we go back to the year when our Society was organized, we find not 

 only pegmatites, but igneous after-effects in general, receiving slight 

 attention in the stuclv of veins. Beainnino;, however, in the early nine- 

 ties and parallel with the spread of petrography, we find them both in- 

 creasing in favor. 



I have no time to trace in detail the very interesting chapter in the 

 history of geology which is based on the earlier contributions to this 

 branch of geologic thought. The chapter covers the last decade of the 

 past century and the first one of the present. Its documentary records 

 largely consist of the writings of J. H. L. Yogt in Norway, 30 and of the 

 distinctive American school of younger men, who were gradually break- 

 ing away from the views of their elders, from the teachings of their youth, 

 and who were striking out into new fields of thought' and interpretation. 

 As one follows through the successive papers of W. Lindgren, J. E. Spurr, 

 and myself as three among the earlier ones who awakened to the dawn of 

 a new era, the growing faith in the intrusive rocks and their after-effects 

 comes out very strongly. Eatlier cautiously adopted at first, we find the 

 converts growing bolder, until a paper was written by myself in 1901 

 whose object was to prove not only that igneous after-effects were more 

 reasonable explanations for most of our veins, but that we then knew 

 enough about the absence of meteoric ground-waters in our increasing 

 number of deep mines and deep bore-holes to show that nothing else than 

 magmatic waters would reasonably answer. 31 



EXAGGERATED ESTIMATES OF METEORIC GROUND-WATERS 



The subsequent shrinkage in the amount of meteoric waters believed to 

 exist within the crust is most impressive. Before 1900 the most fre- 

 quently cited estimate was that of Achille Delesse, in 1861, 32 that the 

 ground-waters, if brought to the surface and distributed uniformly over 

 the earth, would yield a layer 7,500 feet deep. In 1902 C. S. Slichter 

 reduced this estimate to 3,000 to 3,500 feet. 33 In 1903 Chamberlin and 

 Salisbury assign a depth of 800 to 1,600 feet over the surface above the 



