304 
SILUEIA. 
[Chap. XII. 
of which left no doubt of their ancient date, and the still more recent an- 
nouncement of extensive coal-fields in Brazil, in nearly the same southern 
latitude, are striking proofs of the widespread uniformity of these con- 
ditions when the Carboniferous flora prevailed. Professor Agassiz, who 
examined them, declared the Brazilian fossils to belong to the old Carbo- 
niferous age ; and on inspecting the fossil plants from Brazil recently 
submitted to me by Lord Stanley, Her Majesty's Secretary for Foreign 
Affairs, I came to the same conclusion ; whilst Dr. Percy, by analysis of 
various samples of these coals, found their chemical composition and struc- 
ture to be in harmony with the decision of the palaeontologist*. 
In respect to the origin of coal, I still adhere to the idea which I 
broached many years ago, and illustrated by a map, that coal may in some 
instances have been the result of the transport of great matted masses of 
trees and vegetables into marine estuaries (Sil. Syst. p. 152). The ori- 
ginal observation, however, of Logan, that the roots of many of the fossil 
trees are seen to be implanted in the under-clays of the respective coal- 
beds, proves the existence, over very large areas, of coal-growths in situ, 
or, in other words, that such beds of coal were simply subsided jungles. It 
has indeed now become the general opinion of geologists that most of the 
coal has resulted from such littoral growths, in maritime swamps or lagoons, 
of various trees and vegetables suited to these conditions, and that the oc- 
currence of marine spoils among and over the beds of coal has resulted 
from subsidences of that vegetation beneath the sea. It is certainly more 
likely that there should have been subsidences, with periodical pauses, in 
coal strata 12,000 feet thick, like those in South Wales, than that there 
should have been repeated oscillations of the land — descending to receive ma- 
rine productions, and raised again to sustain the growth of terrestrial plants. 
If the plants grew like mangroves in the West Indies, and if the water was 
very shallow, running into muddy lagoons, and full of sand-bars near low 
swampy shores (a condition quite in accordance with what is represented 
in the vignette, p. 286), we should not need to invoke such repeated 
changes of level, but simply appeal to subsidence with occasional pauses. 
In confirmation of this view we have the fact that, with the exception 
of the so-called c Unio-bands,' there is very little evidence of freshwater 
conditions at all ; while the abundance of Spirorbis carbonarius, Leperditia, 
and Beyrichia, occasional bands of Goniatites and Aviculas even in the 
true coal-shales, and the frequent occurrence of marine Shells and shark- 
like Fish point to a condition of things much more like that of the mangrove- 
swamps above alluded to. It is further remarkable that in existing lagoons is 
found a species of Unio nearer in character (according to the late Professor 
E. Forbes) to the so-called Unios of the coal than any true freshwater form. 
In illustrating these views, the geographical conditions under which coal 
has probably been accumulated have been clearly and ably set forth by Mr. 
* The little Pahcozoic Beyrichia has also been found in the shales of the Brazilian coal-measures 
by Prof. Rupert Jones: Greol. Mag. vol. i. p. 132. 
