RECORDS OF MEETINGS OF 1907 343 



SECTION OF GEOLOGY AND MINERALOGY. 

 October 7, 1907. 



Section met at 8:15 P. M., Vice-President Grabau presiding. 

 The minutes of the last two meetings of the Section were read and ap- 

 proved. 



The following program was then offered : 



Alexis A. JuUen, On the Pebbles at Harwich (Cape Cod), Mass., and 

 ON Rude Arrowheads Found Among Them. 



A. W. Grabau, The Sylvania Sandstone — A Study in Paleogeo- 



graphy. 



Summary of Papers. 



Dr. Julien's paper was, in abstract, as follows: Along the south shore 

 of the apron-plain at Harwich the glacial deposits show abundant sections 

 of layers of gravel, often coarse, and at one point huge angular boulders, 

 up to eight feet in diameter, similar to those in the moraine along the north 

 side of the cape. The pebbles consist almost altogether of crystalline rocks 

 in considerable variety, in which, however, three types predominate. The 

 principal one is a coarse binary granite, sometimes porphyroidal, passing 

 by addition of hornblende into monzonite. Its sheared form seems to be 

 represented by pebbles of granite-gneiss or apatite-schist, without mica, 

 and very rarelv of a fine biotite gneiss. 



This rock appears to have been cut by intrusive dikes, both of an acid 

 rock and of one of intermediate character, occurring in abundant pebbles. 

 The one is a pinkish quartz-porhpyry, a white felsite, or finely striped rhyo- 

 lite, whose sheared forms appear to be a white phyllitic gneiss, with minute 

 augen-structure. The other, a rather finely granular gabbro, made up of 

 white feldspar and a greenish black hornblende-like mineral. This rock, 

 by shearing, has passed into a hard greenstone, often decidedly schistose, 

 and perhaps into a banded schist. Besides these three types, several va- 

 rieties of fine crystalline schists, probably metamorphic; rarely small grains 

 of serpentine; and occasional flakes of blue-black argillite. A marked 

 feature in all these rocks is the almost entire absence of mica of any kind 

 and that mineral does not occur even in the sands and clays, at least in scales 

 visible to the naked eye. 



By contrast, the characteristic rocks of the adjoining coast along the 



