176 A. R. Horwood—Aragonite in Middle Lias, Leicestershire, 
fragmentary character of the organisms may denote that the deposit, 
which is very thin but constant, owes its highly calcareous nature 
to its being an ancient sea-floor. The fragments are not rolled, but 
are broken portions that have dropped down into the abyssal depths 
from the higher zones. It seems, indeed, more natural to regard it as 
at first pelagic and gradually elevated, allowing of more littoral 
conditions above. 
The occurrence of calcite and aragonite in the same seam, which is 
found in most instances to be the case where an abundant fauna 
occurs, 1s quite in harmony with known conditions elsewhere, for in 
the hematite deposits at Cleator Moor both calcite and aragonite 
occur. 
It may be of interest here to note the distribution of calcite and 
aragonite when replacing carbonate of lime of shells of Mollusca or 
other animals. Dr. Sorby! has shown that the shells of some 
organisms are preserved as calcite, others as aragonite, whilst in some 
one portion is converted into calcite, the other into aragonite. And 
he demonstrated that certain forms—Foraminifera, Annelids, Echino- 
derms, Polyzoa, Brachiopoda, Ostrea, and Pecten—when preserved as 
fossils have their shells converted into calcite, whilst in Corals, Cephalo- 
pods, Gasteropods (except Patella, Fusus, Littorina, Purpura, ete.), 
Lamellibranchs (except Ostrea, Pecten, and the outer layer of Spon- 
dylus, Pinna, and Mytilus) the shell-layer is preserved as aragonite, 
if not removed by decomposition. Thus the shells and other parts of 
organisms from the base of the Middle Lias up to the crinoidal 
limestone band may be said to be largely preserved in calcite, 
e.g. Ditrypa, Rhynchonella, Terebratula, Pecten, and Ostrea, whilst 
Cerithium, Phasianella, Amaltheus spinatus, Tropidoceras acutum, etc., 
above are preserved in aragonite. Likewise Hodiadema granulatum, 
found above the crinoidal limestone band, when the test is present, is 
preserved in calcite, as are all the fragments of Polyzoa, Pentacrinus, 
etc., in the latter.’ 
In regard to the general character of the shell-layer in deposits of 
Liassic age generally, Sorby wrote: * ‘‘On the whole, the organic 
constituents of the coarser-grained beds of the Lias are closely like 
those of similar beds of Oolitic age, though there is a relatively less 
amount of fragments of aragonite shells and corals. By far the 
greater bulk is made up of joints of Pentacrinus, which in some cases 
constitute nearly the whole rock. Next in abundance are fragments 
of Brachiopods and oysters and shell-prisms; but Foraminifera and 
portions of Belemnites and bone also occur.’’ Further he says: 
‘Occasionally very crystalline, non-radiate, oolitic grains are met 
with, which have all the characters of re-crystallized, small, con- 
centric, aragonite concretions.” 
1 Quart. Journ. Geol. Soc., 1879, vol. xxxv, pp. 63 et seqq. 
2 See also P. F. Kendall, Grou. Mac., 1883, pp. 497 et seqq., for a further list of 
forms preserved in calcite and aragonite respectively in the Coralline Crag. His 
researches bear out in a remarkable way, as to that formation at least, those of 
Sorby, whilst in so far as my own observation has gone I am able to corroborate his 
conclusions in the case of the Middle Lias (vide supra). 
3 Thid., p. 84. 
