Crosby.] 308 [November 5, 
General Meeting. November 5, 1879. 
Mr. F. W. Putnam in the chair. Twenty-nine persons 
present. 
The following papers were read : 
On THE EVIDENCE OF COMPRESSION IN THE ROCKS OF THE 
Boston Basin. By W. O. Crossy. 
So long as the distortion of pebbles of hard materials —such as 
quartzite, granite, ete. — in conglomerate rocks continues to be called 
in question by able geologists, the description of what seem to be 
clear examples of this species of lithological metamorphism will be 
timely and important. . 
Before setting forth the new evidence which has recently come 
under my notice, I will call attention to some general facts tending 
to prove, or proving, the former existence of an efficient compress- 
ing force in the rocks in this vicinity. 
That the Primordial rocks in the Boston basin—the pudding- 
stone and slate — have been powerfully compressed by a force acting 
in a definite direction is proved not only by the folded state of the 
strata (the dips across almost the entire basin being steep and 
frequently alternating), but also by a well-marked cleavage, developed 
in many portions of the slate, and by fractured and flattened or dis- 
torted pebbles in the conglomerate. 
The plicating force, in this region, appears to have operated with 
nearly equal intensity from the south-south-east and north-north-west 
directions, producing flexures which are approximately parallel with 
the axis of the basin; although showing a tendency at most points to 
conform in direction with the adjacent crystalline border. The cleay- 
age of the slate, though nowhere remarkably perfect, is very notice- 
able at many points. Hewitt’s Cove in Hingham, Mill Cove in 
Weymouth, Slate Island, Rainsford Island, and the north shore of 
Squantum are good localities for observing this structure. The strike 
and dip of the cleavage planes are very constant, the former agreeing 
very closely with the average strike of the bedding, while the dip is 
almost invariably to the north at a high angle, 70°-80°. 
Although among the pebbles of the puddingstone hard materials — 
quartz, quartzite, granite, petrosilex, etc. — largely predominate, yet 
comparatively soft pebbles are not at all uncommon. These are fre- 
