16 BULLETIN OF THE 
region west of Pawtucket and Valley Falls on the south side of Black- 
stone River 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 certain 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 difficult 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 their original character, save that they are 
sometimes slightly indented one into the other. The cement of the 
