Clinton and Niagara Strata. — Sarle. 287 
surfaces, and it is further proved by the great size of the con- 
centric masses found in these lenses. This Hfe once estab- 
Hshed, the growth keeping pace with or in advance of the 
slowly accumulating marl sediment, would through a long per- 
iod of time produce essentially this phenomenon. That no 
other element is necessary is proved by masses bearing scarcely 
a trace of any other organism. 
Great detached cores of this material often suggest, from 
their formal resemblance, great heaps of poorly mixed dough. 
Some growths with a fine sedimentary coating filling the pores, 
have the appearance of concretions, an impression which is 
hightened by the structureless character, usually revealed up- 
on fracture. Concretions appear, however, to be entirely lack- 
ing in these lenses. 
The indications are that these masses were not formed by 
the growth of any one colony but that the surfaces were con- 
stantly receiving accessions of new colonies springing up and 
spreading over the old detritus-laden surfaces, some of these 
frequently attaining considerable size. It appears from macro- 
scopic examination of these surfaces, that two or three species 
of Fistulipora are represented, though but one is important as 
builder of these masses. The framework and bulk of all these 
lenses is seen to be bryozoan. 
The irregularities of structure and outline observable in 
those masses is mainly the result of the influence of mechanical 
forces upon the bryozoan growth. Most important of these 
influences was the silt deposited during the muddy-water 
phases, indicated by the shale partings in the eastern portion 
of this area. The prominence of some of these masses un- 
doubtedly enabled the currents to keep them partially free from 
silt, probably facilitated in no small degree by the life functions 
and activities of these living surfaces. Thus where the life 
was vigorous and considerably in advance of the floor deposit 
the sedimentation is seen to have had little effect. This is 
shown where shale layers perhaps three or four inches in thick- 
ness encroach but a little way or end abruptly at the side of a 
fistuliporid growth. [See left side of Fig. i, PI. xxvii.] Any 
loss of territory in this case appears usually to have been 
quickly made good by the more vigorous growth set up with 
the returning clear water conditions. Where the growth was 
