NIAGARA FALLS AND VICINITY I25 



ganisms, such as mollusks and brachiopods, will also find this a 

 convenient resting place. Thus the organically formed limestone 

 masses and the fragmental limestones will interlock and overlap 

 each other around the borders of a growing reef. It follows then 

 that in the neighborhood of the growing coral masses the sands 

 derived from their destruction will be coarser, the finer material 

 being carried farther out to sea, and deposited at a distance from 

 the source. Thus an approximate criterion for the determination 

 of the distance of any given bed of calcareous sand from its place 

 of origin is furnished. If deposits of such calcareous sand are made 

 in shallow water, cross-bedding and ripple marks will be found just 

 as in the quartz sands, and, as we have seen, the former structure 

 is characteristic of most of the strata of Lockport limestone exposed 

 in the gorge section at Niagara. It may be added that, as the 

 organic limestone will continue to form as long as the conditions 

 are favorable, the supply of calcareous sand is practically inex- 

 haustible. Hence thick beds of such lime-sandstones may form. 



In the Xiagaran seas the chief reef-building corals were F a v o - 

 sites, Hal y sites and H e 1 i o 1 i t e s , together with the hy- 

 dro-coralline Stromatopora. Bryozoans also added largely 

 to the supply of organically formed limestone of the various reefs. 

 But perhaps the most important contributors in this connection 

 were the crinoids and related organisms, which may at times have 

 constituted reefs of their own. Their abundance is testified to by the 

 frequent thick beds of limestone, which are almost wholly made up of 

 broken and worn crinoid fragments. The crinoids fell an easy prey 

 to the waves, for, on the death of the animal, the calyx, arms and 

 stem would quickly fall apart into their component sections, and 

 hence yield fragments readily transported by the waves. In the 

 case of the corals and the shells, which latter probably formed no 

 unimportant part of the organic contributions to the reefs, the work 

 of grinding the solid limestone masses into a sand probably required 

 the aid of tools, such as large blocks that could be rolled about by 

 the waves, or it may have been aided by the omnipresent reef-de- 

 stroying organisms. 



The infrequency of exposure of the fossil reefs, which furnished 

 the calcareous sand, need not disturb us. We must remember that 



