GEOGNOSY AND GEOLOGY. 1S 
like the genus Oniscus by means of these rings. Striking forms of trilobites 
are Trinucleus granulatus (fig. 52). Calymene blumenbachii (figs. 53 and 
54, and pil. 42, fig. 60). Phacops downingie (pl. 37, fig. 55.) Brontes 
flabellifer (fig. 56), and the curiously spinous Arges armatus (fig. 57) and 
Asaphus buchii (pl. 42, fig. 61). Trilobites occur in the silurian of North 
America in great numbers, both of individuals and species. Some of these 
are distinguished for their enormous size, as [soteles maximus, which has 
been found in Ohio nearly two feet long and more than one foot wide. 
Remains of fossil fishes first occur in the transition slate, and of the most 
singular and unique character. The characteristic form is that of the 
Cephalaspides, as Cephalaspis Kage (pl. 37 fig. 58), and Pterichthys 
latus (fig. 59.) 
Carboniferous Group (Systeme saisihden Terrain houillier). The 
most important rock species occurring in the carboniferous are grauwacke, 
silicious conglomerate, sandstone, quartz rock, and limestone, which, 
in their petrographical character, are often so similar to those of the 
transition slates, as to be entirely undistinguishable in hand specimens. 
There is, however, a great difference in the occurrence of clay masses. 
Thus while in the preceding formation clay exists as clay slate and burning 
slate, in the present it is found as slaty clay, thus of much less consistence. 
The strata are generally much more extended, and form wide troughs and 
saddles. In some places they have been subject to considerable changes of 
position, being bent, curved. contorted, &c., im the most complicated 
manner: in others, again, the greatest regularity is perceivable. The 
most important metailic salt is the carbonate of iron. Fossils enter in 
large number, some of them coinciding with those of the transition slate. 
The carboniferous system is more restricted than that of the preceding, and 
rarely occupies high levels. In this system three principal groups may be 
distinguished. 
a. Carboniferous or Mountain Limestone (Encrinital limestone), with the 
principal rock species, limestone and dolomite ;, these occur partly crystal- 
line, partly compact and of a prevailing grey color. The mountain 
limestone forms large masses as well as subordinate beds. The cleavage is 
quite decided, and sometimes gives rise to the formation of caves. It is 
often very bituminous, the bitumen being sometimes concentrated in single 
places. The caves often contain tertiary deposits imbedding the bones of 
terrestrial mammalia. Nodulous masses or concretions of hornstone often 
exist in the strata. Slate clay and sandstone occasionally alternate with 
the limestone. Veins occur much less conspicuously than in the transition 
slate: those of galena, in company with fluor spar, barytes, and elalerite, 
are most. frequent. The fossils are generally animal. This group abounds 
in springs. 
~b. Millstone Grit (Flotzleerer Sandstein). This contains principally 
conglomerates and sandstones, of which silicious conglomerates, grauwacke 
clay, and quartz sandstone of a grey color, are the most abundant. They 
are accompanied by a finely laminated, very bituminous limestone, ciay- 
roofing-, alum- and burning slate, as also by silicious shale, jasper, and 
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