ETCHING OF QUARTZ IN THE INTERIOR OF CON- 

 GLOMERATES. 1 



General statement. — Those who have studied the Coal-measure 

 and other highly siliceous conglomerates have frequently reported 

 the occurrence of distinct evidences of etching of the quartz 

 pebbles on certain of the exposed surfaces. The portion 

 removed by the etching solutions may amount to as much as 

 nine-tenths of the original pebbles, though one-third to one-half 

 of their mass are more common amounts (Fig. i). The peb- 

 bles affected ex- 

 hibit strongly 

 pitted and usu- 

 allv verv much Fig. I. — Section showing characteristic outlines of etched 



surfaces. Generalized from surface shown in Fig. 2, A. 



surfaces on the 



exposed parts, while the portions remaining imbedded in the 

 matrix retain unaltered their rounded water-worn characters. 

 The position in which the etched pebbles have been most gener- 

 ally observed is upon the flat upper surfaces of ledges or upon 

 similar surfaces of detached blocks, but instances have been 

 noted, especially in the region about Olean, N. Y., where the 

 etching is also strongly developed on the under sides of over- 

 hanging ledges. So far as known the only published dis- 

 cussion of the phenomena is by Dr. C. W. Hayes 2 who regards 

 the etching as a superficial feature due to the action of azo-humic 

 acid under ordinary atmospheric conditions. 



Character of etching of conglomerate at Blossburg, Pa. — While 

 examining the talus from an outcrop of Pottsville conglomerate 

 at Blossburg, Tioga county, Pa., in 1901, the writer noted certain 

 peculiarities which suggested that the quartz etching might be, 

 in part at least, an internal rather than a superficial feature of 

 the rock. The peculiarities first noted consisted of strongly 



1 Published by permission of the Director of the U. S. Geological Survey. 



2 "Solution of Silica under Atmospheric Conditions," Geo/. Soc. Am., Bull., Vol. 

 VIII, pp. 213-20. 



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