1870.] 47 [Hunt. 



The evidences of the eruptive origin of the granites of our vicinity 

 were well described by Hitchcock, though, as before remarked, he 

 includes with them, under the common name of syenite, many rocks 

 belonging to class A. The coarse white granites on Marblehead 

 Neck are seen in one place intersecting thin bedded and somewhat 

 contorted quartzites, which hold dark micaceous layers, and resemble 

 rather a fine grained gneiss. 1 These beds, which occupy but a small 

 area, are not unlike the strata which at Biddeford, Maine, and in 

 some parts of Nova Scotia, are cut by granites, and probably belong 

 to a newer series than the rocks of class A, as above described. All 

 of these rocks, the granites included, are on Marblehead Neck trav- 

 ersed by dykes of intrusive greenstone, which are sometimes very 

 similar in aspect to certain of the bedded diorites of A. 



Of the rocks of class C, the unaltered argillites of Braintree, hold- 

 ing a primordial fauna, were observed by Prof. Shaler and myself 

 to rest directly upon a hard porphyrinic felsite of the ancient series. 

 The line of demarkation between this and the soft argillite is very 

 distinct. A more detailed examination than we were able to make 

 during a violent rain-storm, will be required to show whether the 

 contact here observed is due to original deposition or to a subsequent 

 dislocation. Reddish granulites directly underlie the black argillites 

 of Weymouth, and the quartzites with conglomerates and argillites 

 of Chestnut Hill Reservoir, and of Brighton near by, are in several 

 places observed in contact with the old dioritic and epidotic rocks 

 already noticed. The Roxbury conglomerate was observed to 

 contain pebbles of the felsite-porphyries, diorites and intrusive gran- 

 ites of the older series, besides, as already remarked by Hitchcock, 

 fragments of argillaceous slate. In this connection may be noticed 

 a remarkable recomposed rock long since correctly described by the 

 same careful observer, as an aggregate of broken-up and recemented 

 felsite-porphyry, (Geol. Mass., pp. 547, 665). He observed it at 

 Hingham and Cohasset, and Mr. Hyatt has since found it on Mar- 

 blehead Neck, resting directly on the parent rock, and very firmly 

 cemented. The unequal weathering of the surface, however, clearly 

 shows both its conglomerate character and the inferior hardness of 

 the cement. Such conglomerates may of course be of very different 

 ages, a remarkable example of a similar reconstructed felsite-por- 



1 These micaceous and gneissic rocks have since been found by Mr. Hyatt to be 

 largely exposed at Naugus Head in Marblehead, where they run to the west of 

 north and are nearly vertical. 



