230 



conglomerate is composed of hard materials, well cemented and 

 firm, the joints cut pebbles and paste alike, so that, considering 

 the coarseness and heterogeneous composition of the rock, the 

 joint surfaces are remarkably smooth and even. This structure 

 is very clearly the effect of unequal transverse or vertical move- 

 ments of the formation, and not of the contraction resulting 

 from the consolidation of the sediments ; nothing but a sudden 

 shearing motion being considered competent to form fractures 

 that are almost mathematically plane in a rock consisting of 

 irregular rounded masses of quartzite, petrosilex, granite, and 

 other hard substances, cemented by a paste of comparative soft- 

 ness, and without regard to the internal structure of these 

 masses. President Hitchcock, in describing the similar, but 

 possibly still more perfect, joint structure of the conglomerate 

 in the vicinity of Newport, Rhode Island, concludes that the 

 rock must have been plastic at the time this structure was 

 developed ; finding evidence of the plasticity in the well-known 

 distortion of many of the pebbles of that conglomerate. This 

 explanation, however, seems entirely inapplicable in the case of 

 the Boston conglomerate ; for where the joint structure is most 

 perfect in this rock there are no indications whatever of a 

 plastic condition subsequent to the original consolidation of the 

 beds. Where the conglomerate is largely composed of soft 

 material, pinite, or kaolinite, as in Milton, the deformation of 

 the pebbles by compression is very evident, the rock being per- 

 manently plastic, as it were ; but it is not here that the jointing 

 is most marked. 



As elsewhere in the Boston basin, veins of endogenous 

 quartz and dykes of exotic basic rocks are of frequent occur- 

 rence in the conglomerate ; and these show a general parallelism 

 with the master joints of the formation. One example of the 

 intrusive masses merits special mention. In Brookline, just 

 east of the amygdaloid, the conglomerate is cut by an immense 

 north-south dyke of coarse diabase precisely similar, as regards 

 its general aspect, its disintegration where exposed to atmospheric 

 agencies, and the formation of boulder-like masses by exfolia- 



