55 



Boston and Providence Railroad, in Hyde Park and West Rox- 

 bury, is certainly mainly of the ancient type. Proceeding 

 eastward, as we mount the bold ridge east of the Clarendon 

 Hills Station, breccia again makes its appearance. Further 

 east it occurs in large patches, but not continuously, from 

 Calvary Cemetery on the north, nearly to Hyde Park Station 

 on the south. It increases rapidly eastward, and covers the 

 most of the area between Back Street and the New York and 

 New England Railroad, the petrosilex appearing at intervals. 

 East of this railroad the petrosilicious character of the Shawmut 

 rocks appears to die out, and they are no longer difficult to dis- 

 tinguish from the Huronian beds. The same is true south of the 

 Neponset, in Milton, except about the eastern end of the more 

 eastern of the two petrosilex areas occurring there ; at this point 

 the breccia reappears. 



In the foregoing attempt to separate the Huronian petrosilex 

 from the Shawmut group I have determined which rocks are 

 Huronian mainly by the following criteria : — 



All compact, flinty rocks, having a clean fracture, and 

 which (to the eye) are perfectly homogeneous (except where 

 porphyritic or banded) , showing no traces of a pebbly struc- 

 ture, were regarded as Huronian ; also all petrosilicious rocks, 

 pebbles of which have been found in the Shawmut breccia. 

 This last test is applicable only where the rock possesses pecu- 

 liar characters by which it can be unfailingly identified. The 

 separation is not, and is not claimed to be, exact, but simply 

 the best that my data will allow. Many of my earlier observa- 

 tions are rendered worthless for this purpose because I did not, 

 at the time they were made, recognize the Shawmut rocks as 

 belonging to an age distinct from the Huronian. 



DETAILS or THE PETROSILEX. 



Petrosilex in Newhury. — The Newbury belt of petrosilex 

 extends along the valley of the River Parker from the mouth 

 of this stream to Byfield Parish, a distance of five miles. The 



