GEOGNOSY AND GEOLOGY. 145 
cave of Montserret, not far from Cordona, in Catalonia (pl. 53, fig. 3), 
and the Grotte des Demoiselles near St. Bauzille du Putoir in the I’rench 
department of Hérault, fig. 1. This so-called transition limestone often 
contains nests of red iron ore, of spathic iron, and the brown iron-stone, as 
also of calamine in company With galena and copper ores. 
Dolomite, both in its true crystalline state and as compact magnesian 
limestone. It is very conspicuous, but limited to restricted areas. It also 
contains caves, which, however, are not lined with stalactites, but with 
rhombohedral crystals. Weathering converts it into a loose sandy earth. 
The extent of the transition slate rocks is often very considerable, as also 
is their occasional elevation above the level of the sea. In the Hartz, this 
elevation is about 2800 feet, in the Black Forest over 4000, and in the Andes 
nearly 22,000. Where the strata are rather level, the mountain forms are 
generally more rounded, while with a more vertical position these are much 
bolder. Clay-tale and chlorite-schist often form widely extended plateaus ; 
and where they are intersected by long valleys, elongated mountain ridges. 
The sides of the transverse valleys, as the Selkethal in the Hartz, and the 
Schwarzathal in Thuringia, are commonly beset with rocks. Granular 
quartz rock and sandstone not unfrequently form rounded mounds, which 
project above the grauwacke and slate masses. Limestone, at low levels, 
presents nothing remarkable in its forms; the contrary is, however, the case 
when it occurs at greater heights; in narrow valleys it forms precipitous 
rocks, which are more conspicuous than those of grauwacke and clay slate. 
The exterior of dolomite is very rough. The Lurleyfels near St. Goarshausen, 
in the valley of the Rhine, consists of grauwacke and clay slate ( pl. 53, fig. 5). 
In England, where the transition slate occurs in great perfection of 
development, three systems have been distinguished; the Cambrian, the 
Silurian, and the Devonian. Hausmann, rejecting the first, only admits 
the two latter, and characterizes them in the following manner : 
a. The older or Silurian System, containing only crystalline species, or 
those approximating to the crystalline. Here belong the older clay slate, 
chlorite-schist, and talc-slate. The clay slate is distinguished from the more 
recent clay-slates, by the presence of a silicate of alumina (chiastolite or 
andalusite) occurring in innumerable quantities of individual crystals in 
some localities. Schulz, however, from recent. investigations, doubts the 
validity of this character. The occurrence of andalusite may be closely 
connected with the penetration of clay slate by granite. Fossils exist only 
in small quantity, and are restricted to individual beds. 
b. The newer or Devonian system, in which the grauwacke, in connexion 
with silicious conglomerate and quartz rock, generally of a reddish color, is 
the most conspicuous member. Limestone occurs in single nodules and in 
connected masses. This system contains a greater abundance of fossil 
remains than the preceding, both of plants and animals. Three principal 
divisions may be established: one containing clay and roofing slate in 
predominance; the second, quartz rock: and the third, grauwacke, with 
subordinate masses of limestone. The clay slate is often fissured, a striking 
example of which occurs on the Lahn ( pil. 49, fig. 8). 
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