Clinton and Niagara Strata. — Sarle. 283 
miles to the westward. The CHnton masses are hmited to the 
upper half of the CHnton upper limestone or projecting into the 
Niagara shale above, but occur at varying horizons within this 
limit ; neighboring masses usually start at about the same base 
level and by far the greater number but a little above the mid- 
plane of the limestone stratum. 
In general these accumulations may be roughly compared 
to greatly thickened lenses. [PI. xxx, Fig. 7.]* However, they 
may be elongated or oblique to the vertical axis. [PI. xxvii. 
Fig 3.] In size they vary considerably. The extremes observed 
range from one to fifteen feet in thickness, and in breadth from 
two to fifty feet, though they may, as cited at Marble creek, 
form an irregular, continuous stratum. 
The strata above and below the lenses are often so strongly 
bent in conformity to these less-yielding masses as to produce 
an arching or doming of the overlying Rochester shale and a 
sagging or basin-like depression which extends into the Clinton 
upper shale beneath. This deformation, taken with the rough 
and irregular structure, gives them the semblance of inclu- 
sions. More careful inspection, however, shows the limestone 
strata on either side continue as a rule into the mass [PI. 
xxvii. Fig. i], where they may double in thickness and the 
bedding becomes lost in the obscure structure. Shale partings 
thin out as they reach the lens or encroach but a little distance 
upon it, [PI. xxvii. Fig i.] with only rare exception appearing 
tc extend through, dividing the mass into upper and lower 
portions. Beds running into the upper portion are traced with 
upper flexure, those into the lower portion with a downward. 
From these facts it would appear that the lenses were pro- 
duced by local thickening of the successive limestone layers in 
the process of their deposition. Hence it is to be inferred that 
their construction was not rapid, but in some instances con- 
tinued through the time when the latter half of the limestone 
was accumulating, and possibly during the beginning of the 
Rochester shale. 
At Rochester this horizon, being a plane of subterranean 
drainage, has suffered considerable mineralization and dissolu- 
tion. This is shown by numerous small cavities spanned by 
♦ The photographs reproduced as plates 27-30 were taken by Mk. Gkorgb 
W. Kellogg, of Rochester, several o.'" them under difficulties. The writer is 
greatly indebted to Mr. Kellogg's photographic skill and intelligent interest. 
