432 SYMPOSIUM ON SEDIMENTATION 



of large thickness of sedimentary rocks. In this work bentonite has been 

 proved to be a decomposed volcanic ash, and as it is known to occur in 

 many zones throughout the sediments of the Colorado and Montana 

 epochs in Utah, Colorado, Wyoming, Montana, and South Dakota, it 

 gives evidence of volcanic activity not previously suspected during these 

 periods. It may be noted that any sediments which contain especially 

 fresh angular feldspar in excess of the quartz should be closely examined 

 as to possible volcanic origin. 



Several years ago Prof. 0. C. Lawson, in the examination of some 

 peculiar clays in the Shinarump group (Juro-Trias) of southern Utah 

 which contained from a trace to twelve cents' worth of gold, was unable 

 to explain the presence of gold in the material. Through the courtesy of 

 B. S. Butler, who was recently able to examine this clay, there can be 

 little doubt that it is a variety of bentonite and a much decomposed 

 volcanic ash. 



The recognition of igneous material in sediments obviously indicates 

 simultaneous volcanic activity in near-by lands or possible seas. It may 

 serve to identify the early stages of batholithic intrusions, for at present 

 we are dependent on the identification of the ages of deformed beds, and 

 this is rarely possible in a satisfactory degree. 



E. W. Sayles : Although De Geer proved to the satisfaction of those 

 with him at the International Geological Congress in Stockholm in 1910 

 that the alternating coarse and fine layers found in Sweden were of a 

 seasonal nature, to find out what actually takes place at the bottom of an 

 active glacial lake in the course of years is of the greatest importance. 

 An apparatus which will collect the year's deposit and yet allow for the 

 action of bottom currents will be necessary. To obtain a core of the 

 bottom sands will also be necessary. To accomplish these results must 

 be the aim of all those interested in this problem. In addition to work 

 in deposits now forming, an examination of the slates and shales from 

 marine formations must be made to find out whether any or many of 

 them show seasonal characters. Physical, mineralogical, and chemical 

 analysis has an extremely important bearing on the whole problem. 



