GEOGNOSY AND GEOLOGY. 109 
The three varieties which have been distinguished possess an essentially 
different character as regards their structure. Common grauwacke is the 
modification exhibiting the components most clearly. To this belong the 
coarse, small, and fine-grained grauwacke, all of considerable solidity. The 
slaty is a finely granular variety, generally of a thick lamination. On the 
surface of lamination, clay-slate and mica not unfrequently . occur 
conformable to the lamination. We must not confound this with grauwacke 
slate, which appears exceedingly like clay slate, and sometimes passes into 
it. The mixture is very thorough, and the lamination less evident than that 
of clay slate. Besides the difference in fracture, the two rocks may be 
distinguished by their mode of cleavage. While clay slate may be separated 
into acutely-angled parallelopipedal pieces, the cleavage of grauwacke slate 
is into ellipso-spheroidal concentric shells. 
Silicious Conglomerate. Rounded or angular pieces of silicious mineral 
are cemented by a siliclous medium. Its hardness and solidity are conside- 
rable ; the predominant colors grey and white. The grain is very various. 
It may be so fine as to pass into sandstone. Pudding-stone is a silicious 
conglomerate in which rounded fragments of flint are cemented by silicious 
matter. 
Nagel-fluh (calcareous breccia). This peculiar name comes from the 
Swiss, and means nail rock. The appellation has been derived from a 
peculiar appearance presented by the weathered rock, in that the pieces of 
cemented matter protrude from the surface, like so many heads of nails or 
spikes. The portions cemented are united by a-medium of similar character, 
only of a finer grain. This rock is distinguished into silicious, calcareous, 
and common breccia (nagel-fluh), as either of these ingredients is in excess, 
or neither predominates. The size of the particles varies considerably, as 
does that of the grains of the cement. 
Calcareous Conglomerate. Blocks or fragments of various rocks, as lime, 
clay, silicious slate, &c., are united by a calcareous cement, often penetrated 
by oxyde of iron. Pieces of lime predominate. The compact or earthy 
cement is sometimes crystalline. 
Shell Conglomerate. A combination of shells, generally broken, or corals 
mixed with quartz or other silicious minerals, united by oxyde of iron, lime- 
stone, or calcareous sinter. The rock is sometimes soft, sometimes hard 
and compact; in the latter case it affords an excellent building material. 
Trachytic Conglomerate. Fragments, generally angular, of trachyte or 
its allied rocks, as pearl, pitch, pumice-stone, and obsidian, are connected 
by a cement resulting from the chemical decomposition and mechanical 
attrition of these same substances. The fragments vary from a diameter 
of several feet to the size of a nut, the latter being most prevalent. This 
rock, on the whole, possesses little solidity; it often contains opal in its 
various modifications. The varieties are: 
Trachytic Breccia, or trachytic conglomerate, with the contained 
fragments, generally angular, and predominating. 
Trachytic Tufa. The opposite here prevails, the cement predominating. 
Here belong some rocks which are distinguished by their color and 
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