
112 GEOGNOSY. = —*([Boox I. ~ 
are highly siliceous, the calcareous matter having been accompanied 
with silica in the act of deposition; others are argillaceous, sandy, 
ferruginous, dolomitic, or bituminous. By far the larger number of 
limestones are of organic origin; though owing to internal re-arrange- 
ment their original clastic character has frequently been changed 
into a crystalline one. Under the present subdivision are placed 
all those limestones which have had a distinctly chemical origin, and 
also those which, though doubtless, in many cases, originally formed 
of organic débris, have lost their fragmental, and have assumed 
instead a crystalline structure. 
Compact, common limestone.—A fine grained crystalline 
granular aggregate, occurring in beds or lamine interstratified with 
other aqueous deposits. When purest it is readily soluble in acid 
with effervescence, leaving little or no residue. Many varieties occur, 
to some of which separate names are given. Hydraulic limestone 
contains 10 per cent. or more of silica (and usually alumina) and, 
when burnt. and subsequently mixed with water, forms a cement or 
mortar, which has the property of “setting” or hardening under 
water. Limestones containing perhaps as much as 25 per cent. of 
silica, alumina, iron, &c., which in themselves would be unsuitable 
for many of the ordinary purposes for which limestones are used, can 
be used for making hydraulic mortar. These limestones occur in 
beds like those in the lias of Lyme Regis, or in nodules like 
those of Sheppey, from which Roman cement is made. Cement- 
stone is the name given to many pale dull ferruginous limestones, 
which contain an admixture of clay, and some of which can be 
profitably used for making hydraulic mortar or cement. Fetid 
limestone (stinkstein, swinestone) gives off a fetid smell (sulphuretted 
hydrogen gas), when struck with a hammer. In some cases the rock 
seems to have been deposited by volcanic springs containing decom- 
posable sulphides as well as lime. In other instances the odour may 
be conneeted with the decomposition of imbedded organie matter. 
In some quarries in the Carboniferous Limestone of Ireland, as 
mentioned by Mr. Jukes, the freshly broken rock may be smelt at 
a distance of a hundred yards when the men are at work, and 
occasionally the stench becomes so strong that the workmen are 
sickened by it, and require to leave off work for a time. Oornstone 
is an arenaceous or siliceous limestone particularly characteristic of 
some of the Palwozoic red sandstone formations. Rottenstone is a 
decomposed siliceous limestone from which most or all of the lime — 
has been removed, leaving a siliceous skeleton of the rock, A 
similar decomposition takes place in some ferruginous limestones, 
with the result of leaving a yellow skeleton of ochre. 
Travertine (calcareous tufa) is the material deposited by 
calcareous springs, usually white or yellowish, varying in texture from 
a solt chalk-like substance or marl to a compact. building-stone. 
Stalactite is the name given to the calcareous pendant deposit formed 
on the roofs of limestone-caverns, vaults, bridges, &c.; while the 
