RHODE ISLAND COAL MEASURES. 183 



In succession and sediments it strongly resembles the beds which appear 

 in East Providence and Seekonk in the middle portion of the coal-bearing 

 series. The following is a more explicit account of the supposed faults 

 which form the block : 



Biake Hiii thrust plane. — If a line be drawn parallel with the Walpole and 

 Wrentham Railroad tracks at Plainville Station and a little west of the 

 roadbed, it will follow closely the boundary between the nearly horizontal 

 strata of the Blake Hill block on the west and the vertical strata of the 

 country on the east (see PL XV), at the extreme western end of the Mans- 

 field syncline. Opposite the Plainville Station this boundary line turns 

 westward in the form of a small loop, inclosing several outcrops of the 

 nearly vertical slaty strata which extend into the area of horizontal beds at 

 the base of the Blake Hill block. It is evident from an inspection of the 

 relations of these two sets of rocks, so sharply contrasted as regards attitude 

 and secondary structures, that the vertical series passes westwardly beneath 



Fig. 26.— Geological section of Plainville Valley and thrust plane. 



the Blake Hill block. Both sets of strata belong to the Coal Measures' 

 but the evidence from fossils so far found is not sufficient to determine the 

 relative position in the normal sequence of beds. 



The Mansfield syncline is overturned between North Attleboro and 

 Shepardsville, and the evidence in the field points to strong pressure 

 exerted from the south and east upon this region. It was probably a result 

 of this pressure that the Blake Hill block was thrust northward upon the 

 edges of the broken Mansfield syncline. The position of the strata in the 

 Blake Hill block in the system of folds before these were broken is hope- 

 lessly lost in the diverse structure of this troubled and now deeply eroded 

 area. 



Faults occur in the Blake Hill block west of the Plainville thrust 

 plane and parallel with its outcrop. They are developed on a small scale. 

 These faults are shown by two classes of facts: (1) The monoclinal ridges 

 of hard sandstone and quartzose conglomerate which strike northwest are in 



