241 



allowance must also be made tor tlie undoubted lateral 

 changes in the character of the strata. In fact, this considera- 

 tion renders the independent correlation of any of the coarser 

 beds extremely hazardous ; and I am fully persuaded that the 

 only reasonably safe clue is afforded by the main belt of slate. 

 This bed, if not folded upon itself, is about 150 feet thick ; 

 and if it is the equivalent of anything in the Village section, 

 it must be the bed of red and gray slate near the middle of the 

 section (6) which passes downward (south) into sandstone 

 and upward (north) into conglomerate and has a thickness on 

 Hersey Street of 130 feet. Assuming this correlation to be 

 substantially correct, the importance of the lateral changes in 

 the strata becomes apparent when we turn to the coarser and 

 more variable parts of the section, and especially when Ave 

 attem})t to find in beds 3, 4, and 5 beneath the slate in the 

 Village section the extended series of strata north and east of, 

 ^. e., below, the slate in the Melville Garden section. This 

 point of view makes it almost necessary to postulate the 

 transverse fault previously referred to as possibly crossing 

 Downer Avenue obliquely between the two ponds. Sup- 

 posing the downthrow to be to the east, as with the faults east 

 and west of Ragged Island, the series of outcrops in the 

 center and eastern part of INIelville Garden become the 

 equivalent of those in the extreme western part of the 

 garden and extending from Downer Avenue along the 

 north side of the main slate. Even then a satisfactory 

 detailed correlation is scarcely possible, perhaps on ac- 

 count of imperfect outcrops, and especially are we at a loss 

 to assign a place to the beds of Langlee Island, without the 

 further assumption of a profound east-west or strike-fault, witii 

 tlic downthrow to the north, between Langlee and Sarah 

 Islands, the beds of Langlee Island being, approximately, or in 

 part, a repetition of those of Sarah Island. This hypothetical 

 strike-fault may or may not be supposed to cross the nortli- 

 south fault between Ragged and Sarah Islands. In the former 



OCCAS, PAPERS B. S, N. H. IV. 16. 



