F. Prime, Jr.—Limonite Deposits of the Great Valley. 485 
The remainder of the slate consists of ferruginous clay, quartz, 
and carbonates of lime and magnesia 
I have noticed that asa rule the fresh ieestinber oe slate can 
only be observed where a mine is being worked; when fresh it 
is white to straw-yellow in color, has a man feel and very slaty 
texture, being composed of minute crystalline plates. As soon 
as work is stopped in a mine the slate commences to decompose 
and becomes rapidly converted to a white or etl clay, the 
latter color being due to oxide of iron. Indeed, in many mines 
no slate can be observed at all, the whole of it being changed 
to clay ibe to the opening of the pits 
ave made a qualitative examination of both white and yel- 
low clays from the same mine as I, and close to the spot where 
the slate was obtained. I found them to contain the same 
ingredients as 
at the damourite-slate. belongs to the Calciferous and not 
to the Laurentian or Huronian periods, i is evident from the fact 
that it is found forming a bed in the limestone immediately 
overlying the Potsdam ‘sandstone and conformably to the latter. 
The ore is always found above the clay or stricted slate, 
or at least in the upper portions of it, never below it ; and 
shiny! in streaks or masses precisely in n the manner noticed by 
Prof. W. P. Blake* at Ocoya Creek, California; and from the 
manner in which the ore occurs it appears to have been de- 
a osited by the percolation of waters containing iron in solution. 
he ore also occurs as so-called * bombshell ore,” and this ore 
when hollow either contains water or the clay resulting from 
the decomposition of the damourite-slate. The interior sur- 
soe of this bombshell ore is oe nently tae apparently by 
thin coating of manganese oxides, a ud often contains small 
never seen the pipe ore in large quantities at any one point or 
associated with the slate or clay 
If this occurrence of the iaucsien slate or clay and ore was 
merely local, it would be scarcely w age noticing; but in fact 
we find that the — or Bolte almost universally asso- 
ciated with the iron ores of the Calciferéus epoch. They are 
therefore of the grentan importance. 
Professor Dana, in a private letter to me, mentions the 
phe of slates with the brown hematite deposit of Rich- 
j Ree County, Massachusetts Professor Shepard,t 
7 See Pacific R. R. Reports, vol. v, p. 1 
+A ace an the Geol. Survey of Suatiad by C. U. Shepard, p. 20. 
