INTRODUCTION 433 



feet, when in Eocene shale, so called, a core was taken after the method 

 outlined above. The driller reports that the barrel was rotated at a 

 speed of about 25 revolutions per minute and estimates the pressure 

 resting on it at about 20 tons. The first 21/2 feet of the core were pene- 

 trated in 20 minutes, but the rest, amounting to one foot, went very 

 slowly and required about 40 minutes. It may be safely assumed that 

 this latter period corresponds wdth that during which significant heating 

 and softening of the drill occurred. 



When the core barrel was raised it was found that the end of it had 

 plainly been softened and twisted, and that it was filled for a length of 

 about one foot by a dark, slaggy plug. Above this came the unaltered 

 sediment, locally much twisted and contorted by the motion of the drill, 

 but elsewhere still retaining its banding little disturbed. The tip of the 

 core barrel and the part immediately above it, sectioned so as to exhibit 

 the nature of its contents, are shown in figure 1. 



Microscopic Characters of the Core Material 



The excellent state of preservation of the core makes it possible to 

 arrive at some definite conclusions regarding the changes that have oc- 

 curred. The unchanged sediment is banded in alternating layers of 

 coarse and fine material, with the coarse markedly predominating. The 

 coarser layers are made up of arkose and the finer layers are more defi- 

 nitely shalelike. Under the microscope the arkose is found to consist 

 largely of angular to subangular grains of the following minerals in order 

 of abundance: quartz, jDlagioclase (mainly Ab2An3),and microcline, with 

 minor amounts of biotite, muscovite, and indefinite claylike matter. 

 The shaly layers are made up largely of finer grains of the same minerals 

 with a moderately greater amount of the claylike matter. 



The rather friable and little compacted material of the unchanged sedi- 

 ment gives place abruptly to the dark slaggy mass of the fused portion. 

 The only change to be noted as the fused part is approached is that a few 

 platy crystals of tridymite have been formed in the arkose. 



The characteristic feature of the fused portion (see figure 2) is the 

 presence in it of glass. In the glass are embedded the undissolved grains 

 of the original minerals, which may vary in amount from about one-half 

 of the whole mass to a very small fraction of it. In places, then, the 

 rock has been converted almost completely to glass, in which case the 

 mass is light colored and of a pale yellow in thin section. As the extent 

 of fusion diminishes, the glass becomes darker and more nearly opaque. 

 The refractive index of the glass varies somewhat, but is never far from 



XXIX — Bull. Geol. Soo. Am., Vol. 34, 1922 



