308 ON THE ANCIENT GRAUWACKE 
principality of Wales, the transition limestones of Ayrshire 
are also gradually forming, in no small part, —so abundant has 
life now become in the waters, — of massive corals, and of the 
stony exuvie of encrinites and molluscs. But to the period 
of this caleareous deposit, —so vastly prolonged, that the mas- 
sive corals of its later ages grow upon rock formed of the re- 
mains of their early predecessors, — there comes a last day; 
a sandy deposit begins to be cast down over it, and, in the 
altered circumstances, many of the corals die, to re-appear no 
more. But life in other forms is not less abundant than in the 
previous time. The sedentary brachipods, — pentamerus, with 
its strange internal partitions, — terebratula, with its perforated 
umbone, — orthis, spirifer, and atrypa, with their long tendril- 
shaped arms, — lie so thickly upon the arenaceous bottom, that 
their remains, as they yield to the inexorable law of death, form 
no inconsiderable proportion of the ever-rising platform on 
which their successors spend also their determined day, and 
yield, in turn, to the destroyer. And thus, during the earlier 
and middle ages of the Caradoc Sandstones, stratum after stra- 
tum is laid down, each, in succession, a home for the living and 
a burying-ground for the dead. And then yet another change 
takes place. The arenaceous deposit is succeeded by a deposit 
of gray argillaceous mud: the fauna, too, alters in at least its 
aspect, in the proportions borne in it by families and genera. 
Though in one certain bed, and for a comparatively short 
period, a small species of terebratula abounds, the brachipods 
generally greatly decrease, —a consequence, mayhap, of the 
altered nature of the bottom, now considerably softer than be- 
fore; but, on the other hand, the cephalopoda, represented 
chiefly by the orthoceratites, very much increase, and the trilo- 
bites attain to their numerical maximum. Scales of fishes, 
somewhat resembling the bony plates on the sides of the stur- 
geon, have been found by Mr. M‘Coy in the Silurians of Ire- 
