Clinton and Niagara Strata. — Sarle. 289 
The presence of silt, where not sufficient to smother the 
growing surface, seems in some cases to have caused the bry- 
ozoan growth to enroll and knot in an unusual degree. 
The growth of new colonies upon these surfaces, the silt 
encroachments and consequent shiftings of growth and concur- 
rent irregular bedding, all combined to produce some very 
complex structures. But the life of these masses passing 
roughly through a maximum to a decline, the resulting form 
is nevertheless always sub-lenticular. 
Currents evidently played an important part in the disposal 
of the lighter material growing upon and about these sites. 
This is proved by the contents of the rim-like thickenings 
sometimes observed, where a limestone tier abuts a fistuliporid 
growth, which was in advance of the plane of sedimentation. 
The thickening (A) on the lefc side of the well-defined fistu- 
liporid head [PI. xxvii, Fig. 2.] is made up of fragments 
of delicate funnel-form and dendritic bryozoans, disarticulated 
brachiopods and crinoidal fragments. The bryozoan material 
growing less and finer a few feet out from the mass, and its 
abundance upon these sites, indicate its source. The crinoidal 
rubble being more abundant farther out upon the slope, sug- 
gests that this material was drifted from outside. Crinoids 
evidently grew very abundantly over the level floor, their re- 
mains contributing more than anything else to the formation 
of the including strata. A remarkable thickening, made up 
mainly of crinoidal fragments, is seen on the eastern side of a 
mass east of the station at Lewaston bights. Currents ap- 
proaching or leaving these masses appear in either case to have 
added to these marginal deposits. Some of the rock formed 
from the light, portable material collected in this way suggests 
Coquina limestone after the fine filling is weathered out. 
An interesting problem is presented by two masses on the 
Genesee which stand so near together that their inner margins 
are scarcely five feet apart. One, six feet in diameter, is com- 
posed almost wholly of fistuliporid material with scarcely a 
trace of crinoids ; the other, twenty-five feet in diameter, and 
starting at the same level, is made up in the peripheral portion 
almost exclusively of crinoidal debris. This suggests that the 
latter had a fringing colony of crinoids. 
