294 The American Geologist. November. i90i. 
tinned for some time to be the center for attached forms. 
Brachiopods are remarkably abnndant in the basal portion and 
comparatively rare in the upper. This mass also contains sev- 
eral trilobite pockets. The accompanying sketch shows rough- 
ly the arrangement of the various elements as displayed in the 
present section of the mass. From the configuration of the 
beds beneath on the right side and their slope, it is evident that 
the portion of the mass broken away extended considerably 
lower on that side. The extinction of the life of the mass prob- 
ably occurred about the time the silts reached the level on its 
sides, indicated by their extension over its top. 
The other cases of growth upward into the shale are along 
Genesee. The upper two or three feet of these masses 
are made up mainly of an irregular, dome-like, fistulio- 
porid growth, discolored with sedimentary matter, of 
the same appearance as that of the enclosing base of 
the Rochester shale. Their life was short after the introduction 
of the shale sediments. 
The conditions for the continued growth of these masses in 
the shallowing turbid waters of the sea at the opening of the 
Niagara epoc'h seem to have been adverse, though more fa- 
vorable in the western portion of the area. The most injur- 
ious was the rapid accumulation of calcareous mud which un- 
doubtedly interfered seriously with the continued local growth 
of the bryozoan life. This effect is shown in the study of 
these masses in the limestone where the incursions of mud- 
bearing currents reduced the growing surfaces which again ex- 
tended on the return of clear water conditions. The same 
fistuliporas in the shale never attained a growth exceeding a 
few inches radius. 
COMP.VRISON WITH EXISTIXG PHENOMENA. 
The fragmental character of the material forming the Clin- 
ton upper limestone at the level of these masses, makes it prob- 
able that these layers were deposited at a depth slightly within 
wave-base. The depth can hardly be estimated from assuming 
a similarity of conditions in any existing sea or bay, for the 
data available on this subject arc found to be discordant. 
It was probably considerably within the one hundred foot line, 
a depth consonant with the more vigorous marine growths of 
to-dav. 
