16 BULLETIN OF THE 



region west of Pawtucket and Valley Falls on the south side of Black- 

 stone Eiver these rocks contain a number of areas of crystalline lime- 

 stone. As yet it has proved impossible to determine the sections in 

 this district with any satisfactory degree of definition. At Mannville 

 on the eastern or right bank of the river, about two hundred feet in 

 thickness of these supposably Pre-Cambrian rocks appear as rather 

 distinct beds of what seems originally to have been clay slate, now 

 changed to a gneissoid material. In these gneissoid rocks near Cum- 

 berland Hill there are extremely interesting mineralogical localities. 

 The remarkable deposit of ilmenite. the iron ore composing Iron Hill, 

 has long received much attention from mineralogists. The hill near 

 Sneech Pond is said to have yielded a cei-tain amount of metallic 

 copper in the various explorations which have been made upon it. 

 Near by, and in the same district, is an interesting and extensive vein of 

 pyrolusite. 



The imperfect sections of this district which have thus far been ob- 

 tained indicate that the total depth of the deposits probably amounts 

 to more than five thousand feet, and may attain to twice that amount. 

 However, as the region has been much affected by mountain building 

 forces, and as the metamorphism has gone so far that little trace of the 

 original bedding is now discernible, it is very diflBcult to obtain a satis- 

 factory account of the series. The nature of the contact between these 

 evidently ancient rocks and those of Cambrian age has not yet been 

 well determined. There are some reasons to suppose that it may be 

 by faulting, but the fact that pebbles of the supposed Pre-Cambrian 

 series are found in the rocks of Cambrian age is rather against this 

 supposition. Actual contact is at no point traceable, the surface being 

 too deeply covered with detrital materials. It may be noted, however, 

 that the line between the two formations is much more direct than 

 that which separates the Cambrian from the Carboniferous, and we are 

 therefore more justified in supposing that faulting may have taken place 

 at this point. 



Whatever be the age of these strata we have termed Pre-Cambrian, 

 it is evident that they were formed long before the Cambrian deposits 

 themselves, and this for the reason that the measure of metamorphism 

 which has affected the two regions is extremely diverse. The Cam- 

 brian rocks exhibit very little sign of metamorphic action. The shales 

 indicate a slight amount of infiltration, and in the conglomerates the 

 pebbles all retain essentially th^ir original character, save that they are 

 sometimes slightly indented one into the other. The cement of the 



