THE OLD RED SANDSTONE. ¥i5 
these again, with but one exception, from the Trilobites of 
the upper Silurian strata; these yet again from the Trilobites 
of the underlying middle beds; and these from the Trilobites 
that occur in the base of the system. Like the coins and 
medals of the antiquary, each represents its own limited 
period; and the whole taken together yield a consecutive 
record. But while we find them merely scattered over the 
later formations in which they occur, and that very sparingly, 
in the Silurian System we find them congregated in such 
vast crowds, that their remains enter largely into the compo- 
sition of many of the rocks which compose it. The Trilobite 
is the distinguishing organism of the group, marrying, if I 
may so express myself, its upper and lower beds; and what 
the Trilobite is to the Silurian formations, the Pterichthys 
seems to be to the formations of the Old Red Sandstone ; 
with this difference, that, so far as is yet known, it is restricted 
to this system alone, occurring in neither the Silurian System 
below, nor in the Coal Measures above. 
I am but imperfectly acquainted with the localities in 
which the upper beds of the Old Red Sandstone unde lie 
the lower beds of the Coal Measures, or where any grada on 
of character appears. The upper yellow sandstone bel is 
extensively developed in Moray, but it contains no trace of 
carbonaceous matter in even its higher strata, and no ot er 
remains than those of the Holoptychius and its contempora- 
ries. The system in the north of Scotland differs as muc 
from the carboniferous group in its upper as in its lower 
rocks; and a similar difference has been remarked in Fife 
where the groups appear in contact a few miles to the west 
of St. Andrew’s. In England, in repeated instances, the 
junction, as shown by Mr. Murchison, in singularly instructive 
sections, is well marked, the carboniferous limestones resting 
