268 THE CAMBRIAN. ' [bull. 81. 



perfectly determined stratigraphic horizons and the fact that there is 

 a thick mass of sediments, usually without fossils and largely argilla- 

 ceous in character, that appear to be pre-Silurian in age. By their lith- 

 ologic character they are correlated with formations referred to the 

 Cambrian group, as found in Maine and Nova Scotia, that have similar 

 lithologic characters and occur in a similar stratigraphic position. All 

 these doubtful formations are in turn correlated by their lithologic char- 

 acters and supposed stratigraphic position with the Cambrian beds of 

 Newfoundland and New Brunswick. 



In speaking of the Mica-schist series of New Hampshire, Prof. C. II. 

 Hitchcock says l that there unquestionably exists an enormous thick- 

 ness of micii-schists in southern New Hampshire above the Huronian 

 series. "No author who has devoted any attention to these groups has 

 suggested an inferior position for them. They may be called Silurian, 

 Cambrian, or pre-Cambrian, according as each author is inclined to re- 

 gard New England, very ancient, or on the verge of the Paleozoic." 



It is doubtful if these exposures should be referred to the Atlantic 

 Coast Province. It is done at present as a matter of convenience. 



EASTERN MASSACHUSETTS. 



The description of the Cambrian rocks of the Boston Basin, by Prof.W. 

 O. Crosby, is the most recent -and the most thorough of any we have, 

 and the summary of our present knowledge of them is taken mainly 

 from his paper. He says: 2 



The oldest rocks which we have found are the Primordial slates and quartzites, 

 and the age of these is certainly and definitely known only at the Paradoxides quarry, 

 in Braintree. We appear to be justified, however, in regarding them, provisionally 

 at least as all of about the same age, partly on account of a general lithologic re- 

 semblance, but mainly because their relations to the different classes of eruptive 

 rocks are everywhere the same. In Weymouth aud Braintree, where we first met 

 these rocks, they are either typical clay slates or slightly calcareous ; but along the 

 northern base of the Blue Hills occasional layers are distinctly siliceous. They prob- 

 ably underlie a large part of the Boston Basin, being covered by the conglomerate 

 and the newer slate ; and north of the basin, as shown in the ninth lecture, they 

 occur in isolated areas among the eruptive rocks. In some ot these areas, especially 

 in the Middlesex Fells and Melrose, and in Woburn, clay slate similar to that in 

 Qnincy and Braintree is repeatedly iuterstratified with quartzite, while toward the 

 southwest, in Natick, and also in Reading and Lynnfield, there are extensive develop- 

 ments of quartzite with little or no slate. It is very clear that the quartzite north 

 and west of the Boston Basin is the source of the quartzite pebbles which play such 

 a prominent part in the composition of the conglomerate, especially in the central 

 and northwestern sections of the basin. In general, the quartzite is more and the 

 slate less abundant northwestward, indicating that the ancient shore line along 

 which these strata were deposited lay in that direction, and originally the Primordial 

 strata were probably spread continuously over all the region to the southeast of 

 that line. 



1 Geological sections across Vermont and New Hampshire. Am. Mus. Nat. Hist. Bull., vol. 1, 1884, p. 

 168. 



2 Physical History of the Boston Basin. Lowell free lectures, 1889-1890. Boston, 1889, pp. 19, 20. 



