ve ee 
Feb. 27, 1873| 
PROFESSOR RAMSAY ON LAKES* 
II. 
I11.— Zhe Waters of the Cambrian and Silurian Epochs and the 
: Lakes of the Old Red Sandstone 
THE lecturer first briefly summarised the reasoning and the con- 
clusions contained in his two preceding Jectures on the origin 
of fresh and salt water lakes, and then proceeded to apply them 
to explain certain phenomena of the above geologi:al epochs as 
deduce from the rocks belongi g to those periods. He was 
about to endervourt» prove that certain of the formations in- 
cluded in a table of the stra‘ified rocks were not formed in the 
open sea as usually supposed, bit were formed in yreat lakes. 
The small numbers of fossil shells fund in fresh-water strata 
and in some cases their total absence rendered absolute demon- 
stration very difficult. The Cambrian strata, seen in England, 
in North Wales, Shropshire, and other parts consist of red and 
mottled sandstones and slates, and contain but few fossils ; trilo- 
bites and one or two fossil shells have been found, not in the red 
strata, however, but in blue and grey shades. Above these the 
Lingula flags (a member of the Lower Silurian) contain fossils 
in great numbers, and of such creatures as must have lived in 
the open ocean of that period. In the Upper Silurian, too, 
fossils are very numerous, and all inhabitants of the sea. But 
above these, in the Old Red sandstone, the numbers have de- 
clined both in genera and species and individuals, and the shells, 
&c., which remain are small and dwarfed in size. Yet the strata 
of the Old Red sandstone lie conformably on the Upper Silurian, 
showing that the passage from the one set of beds to the other 
was gradual, and the change in the fossils is likewise gradual. 
The Old Red strata consist of red sandstones (forming about two- 
thirds of the whole strata) and a red marl developed in England, 
in South Wales, Herefordshire, &c., throughout the whole 
of these strata fossils are very scarce, chiefly occurring in the 
uppermost and lowermost portion. Ia the Ludlow rocks, at the 
upper portion of the Silurian strata, are found fragments of stones 
and seed vessels of land plants for the first time, Not that land 
plants did not exist before that period ; the lecturer thought they 
had done so, but that their remains were not preserved in the 
rocks, inasmuch as they had been formed in the open sea, far 
from land. But their occurrence in the Ludlow rocks evi- 
dently proves that those strata, although truly marine, were 
formed in the neighbourhood of land, and, as the lecturer be- 
lieved, in waters more or less land-locked. These passage beds 
also contain the remains of fish of various genera—Cephalaspis, 
Onchus, &c. But passing upwards into the beds of the Old Red 
sandstone, the fish which occur have their nearest living 
analogies in inland fresh-water areas ; ¢.g. the Lepidosteus of the 
North American rivers, the Polypterus of the Nile, and the 
recently discovered Ceratodus in the rivers of Australia, Large 
crustaceans—Eurypterus and Pterygotus—occur higher up in 
the formation, and in all cases where they are found, and in the 
majority of cases where fish are found in these strata, shells are 
not associated with them. Hence from the absence of shells, 
and their dwarfed forms when they do occur, especially as com- 
pared with the underlying Silurian beds, and from the presence 
of those peculiar kinds of fish, we are entitled to infer that the 
strata were deposited in inland lakes. Again, examination of a 
piece of the red sandstone will show it to be composed of a 
number of minute grains, each surrounded by a thin pellicle of 
peroxide of iron, to which the red colour of the rock is due, for 
when this iron is discharged by chemical means the rock remains 
purely white. All truly marine rocks that we know are in no 
case coloured red ; they are black, blue, green, or yellowish, but 
never red, and there is no reason why in the sea iron should be 
deposited as a peroxide. But in certain lakes in Sweden there 
is a constant deposit of iron oxide ; and though this is due to 
organic agency, still it occurs in lakes, but never in the sea. The 
Old Red sandstone is not a wide-spread formation, but has more 
of a local character. 
The lecturer then showed by means of diagrams how he 
conceived that certain areas of the Silurian seas might be 
isolated and shut off from the ocean by elevation and de- 
pression of the land, that the gradual freshening of these 
inland areas would result in dwarfing and deforming the marine 
creatures in the waters, and rendering them gradually extinct. 
The lakes in the north of Europe, which must have been filled 
~ with sea-water just after the glacial epoch, are similar cases, for 
jn some of them the marine forms are not yet totaily extinct, but 
* Continued from p. 313. 
NATURE 
| of lime and maynesia, often in about equal proportions. 
333 
have become partly acclimatised, and so the occurrence of a few 
such forms in these rocks is no conclusive evidence against their 
being formed in inland and fresh-water lakes, but rather the 
contrary. At the top of the Old Red sandstone beds Jand- 
plants and fresh-water shells occur. In the north of Scotland 
the Old Red sandstone rocks are well developed, the Grampians 
at that time standing out of the waters. The bottom bed is a 
clay with angular b »u'd-rs, much resembling the ‘boulder clay” 
fo: mation, and evidently of glacial origin, and though the lecturer 
dire not assert that glaciers scooped out the lakes in which the 
rocks were deposited, still it was very interesting to have evidence 
of their connection with those lakes. He likewise pointed out 
that if this the »ry of the lake origin of certain formations, first 
suggested by Mr. Godwin Austen, were established, it would 
open out an entirely new field of geological research by revealing 
to us the conditions not only of the ancient seas, but also of old 
continental areas. 
IV.—Salt Lakes of the Permian Epoch 
Prof. Ramsay first explained that it was necessary, in attempt- 
ing to demonstrate the truth of his assertions as to certain 
formations having been deposited in inland waters to commence 
by considering the conditions of the preceding epoch, and trace 
the gradual change in the deposits resulting from the changed 
conditons. He therefore commenced by describing somewhat 
fully the rocks belonging to the carboniferous formation. The 
mountain limestone or carboniferous limestone, which lies at the 
base of the system in the south of England, is a nearly pure 
limestone, composed almost entirely of encrinites, corals, and 
other similar marine forms, and attaining a thickness of two or 
three thousand feet. Above that lies the millstone grit, about a 
thousand feet thick, likewise marine, but containing the remains 
of land plants ; overlying that are the coal measures, consisting 
of an alternation of beds of sandstone, slate, coal, and ironstone. 
In the north of England and Scotland the base of the system 
consists of alternating beds of limestone, sandstone, and shale, 
with occasional beds of coal, and above that the true coal 
measures. The coal was formed by the life and death of Jand- 
plants—Lepidodendron, Sigillaria, Calamites, &c., and under 
every bed of coal is a layer of clay—‘‘underclay,” which is 
nothing more nor less than the ancient soil on which the plants 
grew, and contains the roots of some of the above plants, ¢.2. 
Stigmaria, the root of Sigillaria. The remains of the plants 
accumulated somewhat like existing peat bogs, and were at times 
submerged and covered with a layer of sediment, and again up- 
heaved and oyerspread by vegetation. There must have been a 
large continental area in the latitude in which Britain now stands 
to furnish the gigantic rivers at the mouths of which many of 
these coal-measure forests grew. 
Above the carboniferous strata lie the Permian seen sur- 
rounding some of the Midland coal fields, in a strip from 
Derbyshire into Cumberland, and forming the base rocks 
of a portion of the Vale of Eden. The beds lie unconform- 
ably on or against the coal measures, implying that a 
vast lapse of time, sufficient to allow for the denudation of 
thousands of feet of thickness of strata in some places, took 
place between the deposition of the two formations. At the 
base of the Permian strata lie beds of red conglomerate, sand- 
stone, and marl, known on the Continent as Rothiiegende; and 
above those lie the magnesian limestone. In the magnesian 
limestone a considerable number of marine forms of life occur, 
but compared with the great abundance of those forms in 
the carboniferous limestone, the fauna seems poor and the 
individuals are dwarfed; out of 1800 species of shells in 
the carboniferous genera only 38 genera and 180 species are 
found in the magnesian limestone. The latter strata also 
contain some fossil fish in its lower beds (marl slate), of the 
same genera as those found in carboniferous strata, and some 
show considerable resemblance to those living forms inhabiting 
inland fresh-water areas, mentioned in the last lecture. And 
there are various reptiles found —Labyrinthodont reptiles—which 
were truly amphibious, and which in some cases have left their 
skeletons, but far more frequently their foot-marks impressed 
upon the soft mud of some ancient shore, which likewise shows 
occasionally rain marks and sun cracks. Some of the reptiles 
belong to the Protosaurian genus, closely allied to the modern 
crocodiles or Thecodont saurians, and therefore probably ulti- 
mately connected with the land. And a very significant point 
in regard to the origin of these rocks is their chemical com- 
position, the magnesian limestone consisting of the Spi cae 
Now 
