ABSTRACTS OF PAPERS 89 



The character and relations of the present erosion margin of the thrust de- 

 pend on the character of the rock through which its plane passes along the 

 particular meridian being examined. Furthermore, in many cases, what has 

 been called "Logan's line" is apparently the result of normal faulting which 

 has broken the thrusted mass and that beneath it. The margin or trace of 

 the thrust, as now preserved, is marked by erosion remnants several miles 

 away in the direction of the thrust, apparently even on the New York side of 

 the lake. 



In the Vermont Valley from Bennington to Brandon and along the eastern 

 border of the Champlain lowland, as traced by the writer, there is at some 

 places convincing evidence and at others strong indication of overlap of Lower 

 Cambrian rocks (dolomites and interbedded dolomites and quartzites) on 

 marble and other rocks of probable Ordovician age. It also appears extremely 

 probable that Lower Cambrian terrigenous rocks consisting of schists and 

 phyllites, with interbedded massive quartzites, have been thrust on marble 

 and other rocks, and that in some cases such thrusted masses have been re- 

 garded as belonging to a younger terrane. 



METHOD OF MEASURING AND PLOTTING THE SHAPES OF PEBBLES 

 BY CHESTER K. WENTWORTH ^ 



{Abstract) 



This method has for its object the measurement and portrayal on a chart of 

 the features of the shapes of pebbles characteristic of their mode of origin. 

 It consists in the measurement of the radii of curvature of the sharpest devel- 

 oped edge and the flattest developed face and plotting a simple function of 

 these values on a chart. Pebbles are thus located on the chart according to 

 their shapes, and river pebbles, glacial pebbles, dreikauter, and other sorts of 

 pebbles have each their characteristic positions. This method permits quanti- 

 tative study of the stage of rounding of river pebbles as well as the separation 

 of pebbles of different origins and transitional types and promises to be of 

 great aid in comparative interpretation of measurements of the shapes of 

 pebbles. 



Presented without notes^, with lantern-slide illustration. 



FIELD STUDY OF THE SHAPES OF RIVER PEBBLES 

 BY CHESTER K. WENTWORTH ^ 



(Abstract) 



Measurements of size and roundness were made on upward of six hundred 

 river pebbles in the bed of Russell Fork of Big Sandy River, in southwest Vir- 

 ginia. The pebbles were all of quartzite from an outcrop near the head of the 

 stream and were found at numerous places in the stream for over 28 miles. 

 The increasing roundness is plotted as a function of distance traveled and, 



^ Introduced by A. C. Trowbridge. 



