GEOLOGICAL RELATIONS. 23 



greater antiquity of the gneiss has led to the representation of a folded anticlinal be- 

 tween the two hornblcndic bands in Sudbury. 



The concentric structure of the Boston basin is very obvious, and is shown partly in 

 the section. There is first a grand sweep of gneisses from Andover and Billerica, in a 

 south-westerly and then southerly if not south-easterly direction into Rhode Island. 

 They then pass under the coal-field and the waters of Narraganset and Buzzard's bays, 

 to come up in the New Bedford range. This V-shaped area encircles in similar form a 

 broad but narrower band of hornblendic gneiss and sienite. Within the sienites is a 

 band of compact feldspars and porphyries. A little north-west of Hydepark one mem- 

 ber of this series is a very coarse conglomerate. The section crosses the southern part 

 of this range obliquely, and thus magnifies its thickness. It does not crop out notably 

 next the sienite in Waltham or Watertown, but may be seen in the same geological 

 situation between Melrose and Swampscot. It has been suggested that these felsite 

 beds are of Huronian age. Similar rocks occur abundantly in the New Brunswick 

 areas of this age, though very rarely about the Lake Huron or Vermont terranes. 

 Within the red felsites is a series of Cambrian rocks, showing the characteristic Para- 

 doxides in slates near the base. Below are conglomerates of limited extent; but above 

 is the well known Roxbury conglomerate, used extensively for building purposes in 

 Boston. To the north of this is another set of slates. Prof. N. S. Shaler estimates 

 the thickness of the lower conglomerates or sandstones at several hundred feet. The 

 Braintree slates, together with the lower beds, may be not far from one thousand feet 

 thick. The Roxbury conglomerate is estimated from twelve hundred to two thousand 

 feet and more. The upper slates, — called "Cambridge slates," — are thought to be three 

 hundred feet thick.* All these Cambrian rocks dip northerly, from fifteen to twenty 

 degrees. 



From all that I can learn respecting the structure of this Boston basin, from the 

 writings of J. F. andS. L. Dana, my father's reports, Prof. Shaler's papers, and orig- 

 inal observations of a limited character, in this and the analogous basin of Parker 

 river, f I conclude that the monoclinal dip of the Cambrian rocks about Boston is not 

 altogether the primal condition, but the result of an overturn. If this be a correct 

 induction, the Roxbury conglomerate will be found to be at the summit of the series. 



The next conspicuous set of rocks on the section is the Merrimack series, occupying 

 the valley of Nashua river. The lower portion consists of mica schists of various text- 

 ures and micaceous quartzites, with occasional beds of indigenous granites. In Groton 

 and in Rockingham county, N. H., a bed of soapstone is present. In Harvard the 

 granite is unusually abundant, connected with an interesting conglomerate considerably 

 altered by metamorphism. The resemblance of these beds to what I have thought to 

 be Huronian rocks, near Portland, Me., makes the suggestion worthy of consideration 

 whether the Merrimack group of my annual reports should not be regarded as nearly 

 the equivalent of the Huronian. Hydro-mica schists are rarely met with in the Merri- 

 mack valley,' but the synclinal basin of schists is overlaid by clay slates not unlike 



* Proc. Boston Soc. Nat. Hist., vol. xiii, p. 172. t Proc. Ainer. Ass. Adv. Sci'., i860, p. 118. 



