Dodge.] 418 [Februarys 



tions of localities (including nearly, if not quite, every town from 

 Somerville to Medfield and Hinghani) is a small fraction inside of 

 35° E. of S., local causes have produced occasional strise, whose 

 direction varies to S. 55°, 60°, and even 65° E. 



These, however, mark only the direction of some single pebble 

 movino- on a surface, and the deviation might be caused by internal 

 movements of the ice itself, expanding or contracting. Where the 

 movino* ice was forced against a rock too firm to be either pushed 

 aside or crumbled, and there was easier passage elsewhere, it adapted 

 itself to the shape of the obstacle. Accordingly we find polished 

 and striated surfaces not only on vertical rock walls, but even under 

 underhanging ledges. An instance of the first may be seen in the 

 hio-h face of rock a few rods east of the Clarendon Hills Station of 

 the Boston and Providence Railroad, in the northern part of Hyde 

 Park, while a good example of an overhanging wall is the slate ledge 

 which crops out from under pudding-stone on Murray Street, Newton, 

 corner of Highland Avenue. 



Such obstacles were presented by the numerous outflows of igne- 

 ous rock which, in fact, were almost the only buttresses sufficient to 

 resist the pressure of the ice stream, dividing it instead of being 

 forced from their place and carried along, as were the sienites, pud- 

 ding-stones and slates. And to these intrusive hornblendic masses 

 may be referred the existence, particularly in Newton and Brighton, 

 of numerous ridges of conglomerate which lie parallel to the course 

 of the glacial stream, and transverse to the original ridges of eleva- 

 tion of that rock. The crag and tail formation so common about 

 Edinburgh is, in short, exhibited here, with solid conglomerates in 

 place of the gravels; fit reminders of the solid stream from which 

 they were protected. More or less perfect examples of this may be 

 seen under the lee of most of the larger intrusive masses. From 

 the large ones near Allston Station in Brighton, to Savin Hill, the 

 slope of the conglomerate highlands to the low ground of Charles 

 River marshes, Boston Neck and South Bay, forms a line which can 

 be marked very correctly by a rule upon a map. The smaller cra- 

 dles on land between neighboring eruptives afford by analogy a fair 

 presumption that the larger hollow to the north-east of that line pro- 

 tracted, now filled with water, was also produced by the absence of 

 such protection in that region. 



In regard to the gradual pulverization of the rock fragments torn 

 from their original ledges, it still remains an interesting inquiry 



