THE KATMAI REGION, ALASKA 



593 



to account for the mass that has disappeared. Their presence is 

 not likely to be overlooked, as their color — dark red to nearly 

 black — renders them conspicuous objects among the accumulations 

 of light-gray pumice. Since this explanation will not account for 

 the total mass of rock that has disappeared, two other possibilities 

 should be considered: first, solution of the older rock in the new 

 magma; and second, crater subsidence. 



Respecting solution, it is believed that this process was very 

 active. That this had occurred was suspected several years ago, 

 as specimens of pumice collected by Professor Griggs on former 



Fig. i6. — The crater pit of Katmai. Topographic measurements indicate a 

 diameter of 2 to 2I miles, and height of cliffs as measured from the level of the lake 

 as 2,000 to 3,700 feet. Photograph by J. D. Sayre, 1919. 



expeditions had been examined microscopically by Professor W. J. 

 McCaughey, of Ohio State University, and the presence of basic 

 phenocrysts in the acid magma and the evidences of instability 

 that they manifested had been noted by him. This has now been 

 confirmed independently and much additional evidence has been 

 secured. Various stages of digestion can be followed until the 

 point is reached where quantities of phenocrysts of hornblende, 

 pyroxene, magnetite, and rather basic feldspar are left undissolved 

 in the glassy matrix. In such instances the original groundmass 

 of the basic rocks has been completely dissolved and the pheno- 

 crysts are corroded. Plainly such phenocrysts are out of place in 



