GEOLOGY OF THE COAST DISTRICT. 629 



Orford. Lying upon the west border of the sienite, one might imagine a 

 gradual passage from the one into the other. The distinction between 

 them is very clear in the south-east part of North Andover, where the 

 sienites exist as limited patches of eruptive materials in the midst of the 

 hornblende rock. This, however, is to the west of the proper field of the 

 sienite, as marked upon the map. I have examined several exposures of 

 this hornblende rock in Georgetown and North Andover, and everywhere 

 see that it is a well-defined stratified rock; and it seems to be like the 

 Connecticut valley deposits, which border the ancient gneiss in the same 

 way with this. If we call it Huronian, it would represent the older part 

 of the system, as it is developed in northern New England. It seems to 

 be cut off by the Merrimack group, which has a north-easterly course, 

 while the hornblende runs north and south. 



On my father's map the Parker River area is divided into three parts, 

 the middle called Silurian rocks, the northern porphyry, and the southern 

 metamorphic slates. In 1859 I crossed this basin along the Eastern Rail- 

 road, and suggested in the Proceedings of tJie American Association for 

 the Advancement of Science (p. 118), that the two outer bands might be 

 of the same age, being the corresponding parts of a synclinal fold, the 

 northern dipping at the highest angle and considerably altered from the 

 conglomerates of the southern band. The question of the alteration of 

 conglomerates into porphyry has been often discussed in the meetings 

 of the Boston Society of Natural History, and reported in its proceedings. 

 It is claimed by Mr, T, T. Bouve that the gradual passage of the conglom- 

 erate into porphyry can be satisfactorily traced out in Hingham and 

 elsewhere. T do not feel entirely satisfied with the view, because the peb- 

 bles of the conglomerate are mostly of porphyry, and it would seem there- 

 fore as if that formation had an existence before the sedimentary rock 

 had been manufactured. Elaborate field work will be required to settle 

 the question properly. 



The order of the rocks across this basin is the following, from north 

 to south. At the Devil's Den in Newbury is a mass of serpentine and 

 limestone of Laurentian age, furnishing well characterized Eoaoon from 

 a locality a quarter of a mile to the north. The dip is 30° N. W., and I 

 think there are ledges of gneiss near it. Next are compact porphyries, 

 not very extensive. They are replaced by red and gray argillaceous 



