GEOGNOSY AND GEOLOGY. 1035 
hardness, and silicious particles, which cause an increase of hardness. The 
degrees of hardness are very different, as also the colors, which vary from 
white, through grey, bluish, and reddish, into black. Dhifferent colors 
sometimes co-exist, similar to what is seen in marble, and produced by like 
causes. Carbonates of Fon and manganese are not uncommon constituents, 
which impart to the stone, when long exposed to the weather, a ferrugimous 
or brown crust. 
Limestone shale is distinguished by its fine lamination, rather thick than 
thin, however, and by its extraordinarily fine grain. It is important for its 
use in lithography. - Its principal locality is at Solnhofen, not far from 
Pappenheim. 
The other varieties, the breccious limestone, the columnar, mamillary, 
the cavernous, and the cellular limestones, are sufficiently well characterized 
by the names. The cellular is of a marly character, traversed in various 
directions by a purer mass, which is often calcareous spar, thus producing 
the cellular character. Oolite, or roestone, is an aggregation of globules of 
compact limestone, from the size of a pea (when it is sometimes called 
pisolite) down to very minute particles, the individual particles often 
cohering with extraordinary firmness. 
Calcareous Tufa (travertine). This more or less porous rock, on 
account of its hightness and facility of working, affords an excellent building 
material. The pores depend partly on organic matters, which it incloses, 
and partly they are interstices left throughout the aggregating material. It 
is frequently colored yellow or brown by oxyde of iron or manganese ; 
white, however, generally predominates. 
Scaly Limestone, or limestone with a scaly lamination, is produced by hot 
calcareous springs. It occurs at Carlsbad, and other places. 
Chalk is carbonate of lime in an earthy condition. 
Tripoli (rotten stone). Combination of silex and alumina with lime. 
Light, earthy, stains paper yellowish or greyish-white. 
Marl consists of clay with limestone, and has an earthy, somewhat plane 
fracture. Colors greyish. 
Section 2. Silicious Limestones. 
This comprehends limestones with a greater or less proportion of 
silex. 
Porous Limestone occurs in nature as compact and granular. It is rough 
to the touch, and has the peculiarity of soaking in water greedily, without 
giving afterwards any indication of its presence. 
Silictous Limestone (conite) possesses a variable proportion of  silex, 
and is without the property exhibited by the preceding species with respect 
to water. : 
Chalk Rock. Limestone with a good deal of silex, a little clay, and some 
carbonate of iron. It is important in a technical sense, owing to its 
property of hardening under water, and hence well adapted for submerged 
walls. 
Section 3. Marls. 
These are rocks composed of carbonate of lime with clay, probably net 
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