32 LIAS OF ENGLAND AND WALLS : 
water. Near Coral reefs much calcareous matter is held in sus- 
pension by the sea; and Mr. John Murray remarks that sea-water 
apparently is unable to retain in solution more carbonate of lime 
than is usually found present in it. 
Considering therefore the evidence that carbonate of lime is 
not rapidly dissolved in sea-water, and that we have evidence of 
accumulations of limestone not far distant from land-areas, it may 
reasonably be inferred that under certain conditions there may be 
sedimentary deposits of calcareous mud as well as chemical 
precipitates. These conditions might take place especially when 
the limestone-particles were mixed with argillaceous matter, and 
more limestone was worn from the old cliffs, than could be 
dissolved by the sea- waters. 
Microscopic Structure. 
Microscopic examination of Lias limestones shows the presence 
of granular calcareous matter and of rolled fragments of Crinoids, 
c., in some cases probably derived from older strata, such as the 
Carboniferous Limestone. The Lias limestones, however, are 
f-ometimes more or less crystalline in character, and therefore, 
while regarding much of the material as deposited as a sediment, 
no doubt a certain amount of carbonate of lime was precipitated 
from solution (during consolidation or long subsequently), while 
other portions of the calcareous mud were due to the decay of 
organisms as well as to the voidings of marine animals. 
Dr. Sorby, who has investigated the microscopic structure of 
some Lias limestones, remarks as follows : 
" 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. The somewhat finer-grained beds differ 
mainly in the comparative absence of joints of Pentacrinus, whilst the finest- 
grained were probably derived from completely ground down or decayed shells, 
mixed with much mineral impurity and more or less coloured by bituminous 
matter * * * Grains of glauconite or some other analogous green mineral 
are not uncommon, and it often fills up the hollow spaces in the joints of 
Pentacrinus and in shells, so as to show their structure in a very striking 
manner. Occasionally very crystalline, non-radiate, oolitic grains are met 
with, which have all the characters of recrystallized, small, concentric, aragonite 
concretions."* 
Further reference will be made to the ironstones that show 
oolitic structure. (See p. 300.) It may be mentioned that 
occasionally in the Lower Lias and more frequently in the 
Marlstone Rock-bed, the limestone is " iron-shot." This appear- 
ance is sometimes due to the presence of oolitic granules, but 
more often to tiny organic fragments replaced by iron-ore. (See 
p. 127.) In some instances all traces of original structure are lost. 
* Address to Geol. Soc. 1879, p. 64. 
