DIFFERENT PROVINCES OF NORTH AMERICA IN LATE PALEOZOIC TIME 



59 



ate; both are folded, faulted, and in places considerably sheared. Meta- 

 morphism has gone so far that an imperfect cleavage is developed in the 

 rocks everywhere in the basin. 



In the southern part of the basin, at least, the Roxbury conglomerate is 

 divisible into three members: 



Squantum tillite. 

 Dorchester slate. 

 Brookline conglomerate. 



The Brookline conglomerate lies upon the Mattapan volcanic complex, 

 which is in places interstratified with the two lower members. It is from 500 

 to perhaps 2,000 feet thick and contains some layers or pockets of sandstone 

 and a few thin lenses of slate. 



The Dorchester slate consists of 3,500 feet of slate, shale, and argillite, 

 with some interbedded sandstone and, at or near the top, 40 feet of greenish 

 and yellowish quartzite. Here and there occur beds of reworked tufif. 

 The formation is of a uniform character and appears to have been deposited 

 in a body of fresh water, possibly a lake at the margin of the ice. According 

 to Sayles, the Dorchester slate is composed of " red and purple slates, in part 

 cross-bedded, interbedded with sandstone, and fine-pebble conglomerate. 

 The slate is typically rather coarse-grained and consists largely of reworked 

 volcanic sediments." 



The Squantum tillite is made up of conglomerate and tiUite with some 

 interbedded sandstone and slate. It measures in different places from 50 

 to 600 feet in thickness, but the total thickness is unknown, as the base is 

 exposed in only one locality. It may be that it is separated from the 

 Dorchester slate below by an unconformity and it passes into the Cambridge 

 slate above through 100 feet of transition beds: 



"A large part of the Squantum tillite appears to be of glacial conglomerate, 

 containing striated and facetted pebbles as at Squantum and Hyde Park. * * * 

 He [Sayles] concludes that the ice probably came from the southeast and that 

 there were at least three beds of till with two intercalated interglacial beds; a 

 great piedmont glacier like the Malaspina Glacier must have deposited material 

 such as is found." 



A more detailed account of this important member was given by Sayles.^ 



"The Age of the Roxbury Series. 



"The exact age of the tillite is uncertain. The lithological characters oi the 

 Roxbury series resemble closely those of the Carboniferous and Permian of 

 the Narragansett and Norfolk Basins. The Roxbury series, which consist of the 

 Roxbury conglomerate, the Squantum tillite, and the Cambridge slate, is newer 

 than the Cambrian, as proved by pebbles in it of the granite which cuts the 

 Cambrian. The Roxbury series lies, without much doubt, on the same granitic 



1 Sayles, R. W., Bulletin Harvard Museum of Comparative Zoology, vol. lvi, No. 2, Geo- 

 logical Series No. x, p. 164, 1914. 



