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forty-five degrees, crops on both sides of the intersection of 

 Main and Dexter Streets, in Medford ; and beyond this it can 

 be traced, without sensible change of course, nearly to Mystic 

 Avenue, having an extreme length of more than two miles. 

 The other long dyke is one-fourth to one-half mile north of the 

 one just described. They are not quite parallel, but show an 

 angular divergence toward the west of five to ten degrees. 

 Number two, having the same width and general appearance as 

 number one, first appears near the Mystic River, and for the 

 first half mile it forms the well-marked escarpment which skirts 

 the north side of Winter Hill, and overlooks the alluvial flats 

 of the Mystic. It crops next immediately south of Professors' 

 Row, in Somerville, and is seen for the last time near Curtis 

 Street. 



Many, if not all, of the local irregularities in the dip and 

 strike of the slate are due to the disturbing action of the dykes. 

 In some cases this connection is especially clear. The large 

 Powder House dyke appears to send off a branch toward the 

 west along the north side of Morrison Street, extending across 

 Elm Street, and giving rise to lateral horizontal branches, which, 

 forcing their way between the layers of slate, have lifted the 

 overlying beds to a horizontal position. 



One example of such an intrusive bed is now well exposed 

 in the west side of the quarry. Another interesting phenom- 

 enon observable in this quarry is a north-south crevice, a foot 

 wide, filled with small angular fragments of the slate, cemented 

 by calcite. Toward the south-east the Powder House, or 

 parent, dyke sends off a well-marked branch which extends to 

 and across Willow Avenue, forming a small ridge. On the 

 line of this ridge, on Cedar street, we find, not the diabase, but 

 slate with an abnormal dip — S.E. 10° — as if it were exactly 

 over the end of the dyke and tilted by the same. The action 

 of intrusive beds in elevating and tilting the slates is very evi~ 

 dent where the most southerly of the two long dykes described 

 above crosses Main Street in Medford. 



Assuming the average dip of the Somerville slates as thirty 



OCCAS. PAPERS B. S. N. H. — HI. 17 



