walcott.] SUMMARY MASSACHUSETTS. 271 



According to Dr. M. E. Wadsworth 1 the strata at the trilobite quarry 

 dip a little west of south at an angle of 55°. This is nearly as given 

 by Mr. Isaac Lea in 1858. 2 It is quite probable that the variation in 

 angle of dip mentioned by different observers is owing to the position 

 of the exposures in the quarry, at the time of their respective visits, 

 and by their mistaking the cleavage for the plane of bedding. 



Dr. Crosby 3 describes the rocks at East Point, Nahant, as composed 

 of a very dark gray, almost black, compact slate, with a strike N. 60° E.; 

 and nearly uniform northerly dip of about 40°. The slate is inclosed 

 in, aud extensively cut by, the coarsely crystalline pyroxenic rock of 

 the Naugus Head series. The slate, except where it is vesicular, is of 

 a uniform grayish black color and holds thin beds of limestone aggre- 

 gating, perhaps, 20 feet of calcareous matter. The character of the 

 limestone and its mode of occurrence are precisely like those of Mill 

 Cove, in Weymouth, 13 miles distant. The limestones are interstrat- 

 ified with the slate and not always distinguishable from it. The color 

 is light gray and its texture varies from compact to finely crystalline 

 or saccharoidal. Thin seams and veins of siliceous material become 

 evident on the weathered surface. 



In the interbedded limestones Mr. A. F. Foerste discovered in 1889 

 traces of a species of Hyolithes which he identified as specifically the 

 same as a species found in the decomposed limestones of North Attle- 

 borough 4 He proposed the name Hyolithes incequilateralis, but I think 

 it is identical with Hyolithes communis, var. emmonsi, of the Lower 

 Cambrian horizon at Troy, New York. He states that the Nahant lime- 

 stones are the equivalent of" the red slates in North Weymouth ; and 

 that it is very probable the red slates and included limestones are 

 stratigraphically beneath the Braintree Paradoxides beds. Also that 

 the red slates of Mill Cove, in North Weymouth, are of Olenellus age 

 and beneath the Paradoxides strata. 



The district embracing the Cambrian rocks of Bristol County, Mas- 

 sachusetts, is well described by Prof. N. S. Shaler. He says : 



Several geologists have observed the fact that between Providence, Rhode Island, 

 and Wrentham, Massachusetts, we have an extensive development of interbedded con- 

 glomerates, sbaly slatos, and sandstones, the whole separated from the other detrital 

 deposits of this region by peculiarities of color. * * * Various conjectures have 

 been made as to the age of these deposits. They have been thought by one observer 

 to resemble the Trias, while others, owing partly to their position, have assigned 

 thorn to the Devonian age. 6 



Prof. Shaler discovered fossils of the Lower Cambrian or Olenellus 

 zone in this series of rocks in 1883, which for the first time gave data 



1 On the relation of the Quincy granite to the primordial argillite of Braintree, Massachusetts. Bos- 

 ton Soc. Nat. Hist. Proc, vol. 21, 1883, pp. '274-277. 



2 [On the trilobite formation at Braintree, Massachusetts]. Phil. Acad. Sci. Proc, vol. 9, 1858, p. 205. 

 8 Contributions to the geology of Eastern Massachusetts. Boston Soc. Nat. Hist-. Occasional papers, 



No. 3, 1880, pp. 195, 261, 262. 



4 The Paleontological horizon of the limestone at Nahant, Massachusetts. Boston Soc. Nat. Hist. 

 Proc., vol. 24, 1889, pp. 261-263. 



•On the geology of the Cambrian district of Bristol County, Massachusetts. Bull. Mus. Comp. Zool. , 

 Harv. Col., vol. 16, 1888, pp. 13, 14. 



