290 The American Geologist. November, i90i. 
Where sedimentation was nearly apace with the bryozoan 
growth, the fistuHpora, by repeated extensions over the frag- 
mental matter collecting upon its margins, often formed an in- 
terweaving so complete that the passage from the mechanical 
deposits to those of life-growth is difficult to determine. 
In the masses where the lighter material was particularly 
abundant, a slight shifting has resulted in an imperfect layer- 
ing, and sometimes encumbering the surface which must have 
interfered nearly as seriously with the fistuliporid growth as 
did the silt. 
From the small size of these masses and the manner in 
which the lighter material is disposed about them it is seen 
that they could not have been an important source for the sup- 
plies of organic matter which compose the including limestone 
strata. 
Though many forms of life appear to be especially associ- 
ated with the peculiar conditions found upon these sites, yet 
many others found here are cosmopolitans. The level floors 
were evidently richly covered with life, though not comparing 
with the luxuriance of these tumuli and their immediate neigh- 
borhood. Some forms seem to have preferred the level 
stretches just as others preferred the fistuliporid sites. It is 
probable however that with favorable conditions for preserva- 
tion the level floor would be found to include them all ; as pri- 
marily it must have been the source. More extensive collec- 
tions will do much to clear this point. When we compare the 
contents of different layers of a stratum which vary consid- 
erably among themselves in their assemblage of forms with the 
contents of the lens-rock proper we find that the principal form 
or forms of a given layer are oftentimes not shared by the 
lenses at or above that particular level. For example in Roch- 
ester a ten-inch layer, the thickest in the upper six feet of this 
limestone, abounds in Whitfieldclla naviformis. This layer 
abuts, and in many cases fuses with, lenses, yet rarely is this 
brachiopod found in them. 
Weathering brings out the organic features though slowly 
and superficially. A lens partially blasted away in 1885, in the 
extension of a sewer down to the water level of the Genesee, 
has weathered so far as to show how completely the mass is 
made up of FistuHpora, but under a blow of the hammer, the 
