124 Canadian Record of Science. 



being deposited, are, to a large extent, inimical to the 

 existence of abundant organic life, but in the clearer 

 waters beyond these zones, provided there is a sufficient 

 food supply, various forms will normally, be more abundant. 

 These types, by the growth of their hard parts and the 

 subsequent comminution of these, transfer the lime from 

 a solution of calcareous compounds in the sea water to the 

 ocean floor to eventually form limestones. Again, under 

 certain conditions the calcareous materials are deposited 

 without the intervention of organic forms. For the 

 formation of limestone beds in these ways deep water is 

 thus not always essential (and, in the former case, beyond 

 a certain maximum depth, fixed by the depth of water at 

 which a fauna can thrive, detrimental). So long as the 

 lime-producing forms are not hindered in their growth by 

 deleterious material or lack of food supply, they will 

 continue to grow. Hence, under favourable oonditions, 

 there will be frequently formed off-shore reefs, usually 

 chiefly of corals, but many other forms will also thrive 

 here, e.g., Barrier reefs of Australasia. These reefs may 

 also be covered with comparatively shallow water, and the 

 older parts might in places be exposed. Under these con- 

 ditions, the action of the wind and waves will lead to the 

 comminution of the materials of which they are composed 

 and the formation of all the various types of limestone 

 rock, included within the types recently designated 

 calcirudite, calarenite, calcilutite by Grabau. 1 The dis- 

 tribution of these various types of rock will bear the 

 same relation to their source, the coral reefs, as does the 

 normal distribution of sedimentary formations to their 

 source, the oldland. Such conditions seem to have pre- 

 vailed during the period of the formation of the Silurian 

 and the Devonian limestones in New York and elsewhere. 

 Still another secondary consideration now suggests 

 itself. If the supply of material detrimental to the exist- 

 ence of a great abundance of organic forms diminishes or 



1 Bull. Geol. Soe. Amer. Vol. XIV., 1908, p. 349. 



