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III— TIIE GENERAL MICROSCOPIC STRUCTURE OE TIIE 
FORAMI.NI FERAL LIMESTONES OE POKOLBIN. 
These rocks contain numerous large fossils, visibly crowded together on the 
surface of weathered hand-specimens. Even by the unaided eye, the 
foraminifera may be easily detected, as their tests, especially in Nubeovlarla, 
present a whitish surface. In thin sections under the microscope the general 
appearance of the rock is that of an impure limestone, largely composed of 
calcareous organic fragments such as shells and polyzoa, bound together in 
part by a fine calcareous and silty mud. Quartz grains occur in the rock ; 
they are very minute, and not at all abundant. Cavities seem to have been 
left between the fossils during the accumulation of the stratum, but the 
spaces have since been completely filled in with crystalline calcareous material, 
chiefly in the form of calcite. At the period of cementation, crystallisation 
took place around the separate fragments, much as they do at the present 
time around the pieces of coral and calcareous sand of a coral beach, until 
the interspaces were entirely filled up, either by primary aragonite crystals 
or by slender scalenohedra of calcite. 
Occasionally the finer structure of the ground-mass of the Pokolbin 
limestone is granulated, as though incipient oolitic structure was being 
formed by chemical deposition. The composition of the rock is sometimes 
varied by the inclusion of fragments of igneous rocks, which are too 
decomposed for much to be said as to their nature, except that they appear 
to be mainly of an andesitic type. 
The included organic fragments in the limestone are referable to Pora- 
minifera, Radiolaria, Sponges, Crinoidea, Polyzoa, Brachiopoda, Pelecypoda, 
Gasteropoda, and Ostracoda. The molluscan shells have, in many cases, been 
bored by perforating Eungi or Algae. Through the mass of the rock there 
is disseminated a good deal of peroxide of iron, chiefly in the form of tiny 
blood-red granules. 
