COAL MEASUBES OF THE DEDHAM QUADEA]^5"aLE. 189 



These beds dip to the S. 30^ ; the cleavage dips N. 50^. The exposure 

 is noteworthy in exhibiting preexisting dark slates, broken up and deposited 

 at this time. The slate fragments are angular and conspicuous elements in 

 the layers in which they occur. They vary in length from 3 to 4 inches 

 and in thickness from 1 to 2 inches. The attitude of the fragments and the 

 unruptured state of the stratum, except for joints and cleavage, preclude the 

 formation of the slate fragments by the disruption in post-Carboniferous 

 times of an original argillaceous layer. For similar reasons, the fragments 

 are not to be regarded as contemporaneous pockets of argillaceous sedi- 

 ments. In the absence of contained fossils or other evidence of the age 

 of these pebbles, there is doubt whether they are fragments of the 

 subjacent Carboniferous shales or are detached pieces of the dark slates of 

 middle Cambrian age, remnants of which occur at Braintree, in the Boston 

 Basin. The not infrequent occurrence of signs of contemporaneous erosion 

 in the Carboniferous beds in the basin leads me to conclude that the con- 

 glomerates are of the class called ^4ntraformational" by Walcott.^ I have 

 already described a limited occurrence of this kind at the contact of red 

 and gray beds in Attleboro. North and east of the junction at Mansfield 

 occur a few outcrops of conglomerate with small pebbles and associated 

 sandstones. At the junction are two exposures of grayish feldspathic sand- 

 stone — massive beds, like the typical sandstone ridges of the Seekonk 

 formation. The knob west of the railroad carries the flattened impression 

 of a large tree, and the rock in the railroad cut is much shattered. 



One and a half miles south of the Junction is a locality where coal was 

 formerly mined (see PL XVI). In the absence of surface exposures, and 

 because of the abandonment of the old shafts, it is impossible to get other 

 data concerning the structure at this locality than those furnished by the 

 records of earlier surveys and by one recent boring. From the Massachu- 

 setts report^ it would appear that the beds here strike NW.-SE, and dip 

 NE. from 30° to 35°, or as high as 45°, and in another place that the 

 strike of the beds is NE.-SW.; but the observation of C. T. Jackson,^ that 

 the "strata between which the coal beds are included run quite uniformly 



iC.D. Walcott: Bull. Geol. Soc. Am., Vol. V, 1894, pp. 191-198 j Bnll. U. S. Geol. Survey No. 1.34, 



1896, pp. 34-40. 



2 Final Report on the Geology of Massacliusetts, 1841, pp. 183, 540. 



3 Geology of Rhode Island, 1840, p. 107. 



