Clinton and Niagara Strata. — Sarle. 
293 
LENSES SURVIVING IN THE ROCHESTER SHALE. 
All the lenses observed, with possibly four or five excep- 
tions, terminated with the limestone of which basally they are 
always a part. One exception is the mass at the third watch- 
man's hut. [PI. xxix, Fig. 6. J, This starts with a thicken- 
ing of perhaps the upper two feet of the limestone, of which 
the capping ten or twelve inches is of darker color and made 
up of a mass of drifted brachiopod shells and other remains of 
Niagara type. This stratum is slightly thickened on the right 
side of the lens and thins out beneath its edge, while on the 
left side it fuses into the lens. A limestone parting three or 
four inches thick in the Rochester shale above and midway on 
its sides, also fuses into this mass. This lens carries several 
inclusions of shale, apparently of the same character as that 
abutting its sides and arching over its dome-shaped top. These 
may have started as surface pockets like those in the top of 
the same mass [See accompanying sketch], which might 
Bfifozocins (mainly /^/yf-ciipora ) gg S>hale 
■r^iiobii-ea nnD Ce-phalopods 
aracfiiopacls <.otzA<7Tici/LA-reD) gij Crinoids 
Driff-matOEr, fine niix^cL, D. 
very naturally have become enclosed in the same way 
had the growth continued longer. These facts point to the 
growth of the mass contemporaneously with the deposition 
of the surrounding Rochester shale. 
This mass was one surviving the limestone period. The 
species of brachiopods and bryozoans found are mainly 
forms known to have lived on into the shale period. This 
limestone dome projecting through the silt flow, probably con- 
