SHALEK.j 
INTRODUCTION. 
15 
sists of a patch of limestone about fifteen feet thick, which has been 
converted into siderite by the iuleaehing of iron-bearing waters from 
the ferruginous Ohio (Devonian) shales which formerly overlaid the 
bed. Since the escarpment of the Ohio shales retreated beyond this 
bed it has been subjected to oxidation and is now in the main converted 
into a much decayed limonite. Beneath this limonite there is a green- 
ish, argillaceous sand which contains frequent nodules of lime phosphate. 
These nodules are smooth-surfaced and not unlike some of the nodules 
from the Carolina district. They contain as much as 92 per cent, of lime 
phosphate. It seems likely that these nodules were formed by the leach- 
ing out of the lime phosphate from the overlying ferruginous layers, 
which has completely removed the lime carbonate, but has not removed 
the whole of the less soluble lime phosphate (Fig. 1). 
Fig. 1. Section at Olympia, Bath County, Kv. (Preston ore bed). A, soil; B, limonite iron ore,- 
C, siderite iron ore; D, phosphatic nodules. Scale: 1 inch = 12 feet. 
Although this deposit of nodules is not of sufficient abundance to have 
any economic value, it is clear that we have in it an indication of a method 
where, by a slight variation of the conditions, important beds of nodular 
phosphates might be found. 
In the horizons of the Oambro-Silurian section, or, as it is generally 
called, the Lower Silurian, there is much greater reason to expect the 
occurrence of workable phosphates than in the beds immediately above. 
It is likely that the most important of the Spanish deposits belong in 
strata of this period, and the Welsh deposits of this general age are of 
noteworthy extent. We know, moreover, that the commoner marine 
animals of this part of the geological section were particularly adapted 
for the secretion of lime phosphate. 
The search of this portion of the section for phosphates should be 
directed to two ends: first, to finding beds of very phosphatic limestone; 
and, second, to discovering veins formed by a segregation of lime phos- 
phates either in the form of the Spanish deposits referred to by Dr. 
Penrose or in the condition of nodular accumulations, The areaof rocks 
of these Lower Silurian and Cambrian periods in this country is very 
extensive, and so far there has been no search of them for phosphatic 
materials. The little work done in Kentucky during the above men- 
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