SILURIAN SYSTEM. 157 
IV.—Silurian System. 
Description. —The Silurian System contains rocks less changed by heat 
than those below, and exhibiting more abundant life. In those already 
mentioned, the change that has passed over the strata has been so great as 
to render it difficult to say, with certainty, how the rocks were originally 
formed, but henceforth all hesitation vanishes. We have slaty sandstone 
finely laminated, and often exhibiting ripple-marks ; conglomerates chiefly 
of rounded pebbles, clays, and limestones, with corals and other fossil 
remains, all of great thickness. The system has received the name of 
Silurian from being very fully developed in a part of South Wales 
anciently called Stluria, and is therefore named from its chief locality. 
From these rocks are obtained roofing-slates, freestone for building, 
flagstones for paving and other purposes, limestones, from which lime is 
got by burning, and valuable ores of lead, copper, silver, mercury, and 
gold. 
Organic Remains.—Parts of the stems and leaves of water-plants and 
club-mosses, and a few sea-weeds, are found, but all scarce and much 
broken. No land animals have yet been obtained, and though it would 
be rash to say that they do not exist, everything seems to render this very 
probable. But marine fossils are numerous and well marked, some of 
them being very beautiful. We find corals of different kinds, named 
according to their appearance, such as the sun, star, cup, pipe, chain, 
spider, and honeycomb corals. One of the commonest forms in the 
Silurian rocks is a very beautiful curved creature like the plume of a 
goose-quill, called the Graptolite,’ from looking like a pen on the rock ; 
some single, others double, some 
straight, others beautifully spiral. 
Another very abundant form is the 
Encrinite, a coral creature more nu- 
merous in the Carboniferous System. . 
We find also star-fishes, and numer- 
ous shells with single and double 
valves, some like the periwinkle = 
and cockle being abundant. But Coral. Trilobite. 
the creature that swarmed most in Fig) 84. :Silurian Fossils. 
the ancient Silurian seas was the ‘ 
Trilobite, so called from its body consisting of three lobes or divisions, 
above which was set its head with large double eyes, still to be found 
entire. It had various forms, and seems to have been very active, and 



VD) 0 ee 


1 From grapho, I write, and lithos, a stone. 
, 
