DAVIDSON — SCOTTISH CAItliON'lFKROLIS lUiACIIIOl'ODA. 
4G3 
general reader may better understand the position in tlie series occu- 
pied by the species to be hereafter described. 
It is well known to every one possessing a knowledge of the first 
rudiments of geology, that the Cai-boniferous system lies between 
the Devonian and Permian formations ; but we cannot expect always 
and everywhere to find the sequence complete. Instances ai-o not 
wanting wherein Carboniferous strata repose directly upon Silurian 
or older rocks, and are overlaid by Jurassic or still younger deposits ; 
but in such cases, which are likewise common to formations of all 
ages or periods, the natural order of succession does not exist, for the 
strata which should underlie or overlie in natural order are wanting 
from some cause or another. 
The rocks wliich compose the Carboniferous series are not every- 
where exactly similar ; for in some districts a certain bed, or series 
of beds, may be largely developed, while they may be attenuated, or 
entu-ety absent, in another. The Carboniferous system is made up 
of a vast accumulation of conglomerates, sandstones, shales, iron- 
stones, limestones, and coal-seams, and certain portions of the system 
present a marine, others an estuary character, while a thirtl is entirely 
composed of terrestrial vegetation ; and, as stated by Mr. Page, in 
his excellent text-book, " the frequent alternations of strata, and the 
great extent of our coal-fields, indicate the existence of vast estuaries 
and mland seas, of gigantic rivers and periodical inundations, while 
the mountain-limestone, with its marine remains, reminds us of low 
tropical islands fringed with coral-reefs, and lagoons thronged with 
shell-fish and fishes." 
Before alluding to the divisions that have been proposed for the 
Carboniferous system, we must briefly notice that, although Sir R. 
Murchison and the generality of geologists have pronoimced the Old 
Red sandstone of Scotland* to be the full equivalent in time of the 
Devonian rocks of other countries, some geologists, such as Professor 
de Koninck and Mi*. Kelly, have suggested that these red sandstones, 
especially in the southern portion of Scotland, and in Ireland, should 
be considered as forming part of the Carboniferous series. Be this 
as it may, the celebrated author of the " Silurian System" has him- 
self admitted that, " as we approach the summit or higher beds of 
the Devonian or Old Red sandstone, we are gradually introduced to 
the fauna of the Carboniferous era." Therefore the passage between 
the upper beds of the Old Red sandstone and the lower one of the 
Carboniferous system has been gTadual ; while the same may be said 
relative to that which connects the upper beds of the Carboniferous 
with the lower ones of the Permian formations. Some geologists 
would, therefore, place the basement-line of the Carboniferous system 
lower down than others appear disposed to admit ; and hence the 
difference in opinion that may be traced in the various sections or 
* The rock is not eveiy^vhere of a red colour, there being also enonnous beds 
of yellow, whitish, purplish, and rusty-coloiu-ed sandstone, with coarse con- 
glomerates, and dark-gi'ey micaceous flagstones. 
