F.. Prime, Jr.— Lower Silurian Fossils, 263 
When exposed to the weather they very rapidly decompose to 
soft unctuous plastic clays in a few days, and some of these 
will in time probably become valuable in the manufacture of 
coarse kinds of pottery. Generally they contain more or less of 
the carbonates of lime and magnesia, and silica, mixed with the 
damourite.* Hydromica slate also occurs the greater portion of 
the distance from the western boundary of Lehigh county to 
the Delaware River, at the junction of the No. IT limestones 
with the No. III slates, here also carrying brown hematite ores 
in extensive deposits. 
It also occurs intercalated in the limestone, forming layers 
from the thickness of a sheet of paper to several feet, and these 
layers are innumerable. Their existence has been seen both in 
rock outcrops as well as in wells which have been sunk. 
The clay to which the hydromica slate decomposes is gener- 
ally of a white color, although sometimes brown from the pres- 
d 
Overlying the hydromica slates, and conformable with these and 
the Potsdam sandstone, is the No. II or Magnesian limestone ( u- 
roral of Rogers), which extends as a great inass varying Jrom six to 
It disappears, however, in the upper stra 
The limestone varies fr i ? ‘ 
being for the most part compact to semi-crystalline, while there 
are occasionally shaly beds. In composition it varies much. 
often approaching a true dolomite, again a pure limestone. But 
from the isolated analyses made it would seem as if or per- 
centage of magnesia was less in the upper beds than the lower 
ones. The limestone is always siliceous, often very much 
and hence much care is now being taken by many of the iron- 
i i s of it, which are low in silica, so as to 
masters in selecting beds of it, whic Oo nak vdeo 
Tt often contains minute 
i i : f Penn., p. 12. 
* Report of Progress for 1874 of Lehigh Dist. Geol. Survey : 
+ See Geology of Tennessee, by Safford, pp. 215, 218. 
