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Parr Il. § vi] FRAGMENTAL ROCKS—ORGANIC. 167 
in lakes and ponds by the accumulation of the remains of shells and 
Entomostraca on the bottom. When such calcareous deposits become 
solid compact stone they are known as fresh-water (lacustrine) lime- 
stones. These are generally of a smooth texture, and either dull white 
or pale grey, their fracture slightly conchoidal, rarely splintery. 
Caleareous (Foraminiferal) Ooze—a white or grey cal- 
eareous mud, of organic origin, found covering vast areas of the floor 
of the Atlantic and other oceans, and formed mostly of the remains 
of Foraminifera, particularly of forms of the genus Globigerina. 
‘Further account of this and other organic deep-sea deposits is given 
in Book III. Part II. Section ii. 

Fié. 28. —FoRAMINIFERAL (GLOBIGERTINA) OOZE, DREDGED BY THE “’ CHALLENGER” EXPEDI- 
TION IN Lat. 50° 1’ S., Lone, 123° 4’ E., FRom A DEPTH OF 1800 FaTHOMS, MAGNIFIED 
50 DIAMETERS. 
Shell sand—a deposit composed in great measure or wholly 
of comminuted shells, found commonly on a low shelving coast 
exposed to prevalent on-shore winds. When thrown above the reach 
of the waves and often wetted by rain, or by trickling runnels of 
water, 1t is apt to become consolidated into a mass, owing to the 
solution and redeposit of lime round the grains of shell (p. 155). 
Coral-rock—a limestone formed by the continuous growth of 
coral-building polyps. This substance affords an excellent illustra- 
tion of the way in which organic structure may be effaced from a 
limestone entirely formed from the remains of once living animals. 
Though the skeletons of the reef-building corals remain distinct 
; é =) 
on the upper surface, those of-their predecessors beneath them are 
