270 
GEOLOGY OF THE SECOND DISTRICT. 
Fucoidal Layers, and Calciferous Sandrock, 
In the county of Essex, we do not find circumstances favorable for the full development 
of all the members of the Transition rocks. From the limited area which they occupy, from 
their proximity to the Primary rocks, and from their being necessarily but the outskirts of a 
few lower members of this series, it is not expected that they will present continuous and 
Avell characterized masses except in a few localities; and moreover these limited areas will 
be frequently imperfect, in consequence of the entire absence of some members of the series, 
their thinning out, or disappearance by denudation. 
The mass which has been designated under the term Fucoidal layers, is thin wherever it 
occurs in this district, and it is particularly so in this county. This mass is only distinctly 
developed at Port Henry; and even here, it is difficult to separate it from the calciferous 
sandrock; and the same difficulty is not uncommon elsewhere, as the conditions required for 
making the distinction are not favorable. When slaty layers are interposed between the pots- 
dam and calciferous sandrock, the fucoidal layers appear distinct and well defined; but they 
also exist where the passage from the lower to the upper rock is effected by a gradual substi¬ 
tution of calcareous for siliceous matter: the fucoids still occupy the intermediate position 
between the two rocks, but the layers are then thicker ; the marine vegetables do not appear 
so numerous as where argillaceous matter is present, and the mass merely appears worm eaten, 
or perforated by cylindrical holes, where the fucoids are entirely removed by decomposition, 
or the surfaces show the remains of vegetables in the condition of external crusts, and with¬ 
out any visible vegetable structure. The usual appearance of the fucoids in these layers is 
that of a bundle of sticks, both curved and straight, lying, however, in various directions. 
No appearance of foliage or leaves is visible upon these layers. We find them in all stages : 
some very distinct and prominent; others obscure, and distinguished with difficulty from ridges 
and elevations produced by other causes. 
The fucoidal layers are rarely more than fifteen feet thick in the second district; and not¬ 
withstanding their apparent insignificance, they are remarkably persistent: they are rarely, 
if ever, absent. Considered as fossils, they are too imperfect generally to be identified by 
figures, still they are easily recognized when once pointed out. They have, however, suffi¬ 
cient character to show themselves distinct from other marine plants belonging to the same 
family. On a careful examination of the rocks above, Ido not find any traces of them : they 
therefore appear to be confined to the period of the calciferous sandrock ; and should this 
prove to be a fact, it will give us an additional means for recognizing the position and relations 
of the strata by the earlier vegetable remains. 
I will remark in this place, that vegetable remains seem to be quite as useful in identifying 
rocks, as those of animals ; and as we find them widely distributed upon the same platform, so 
we should also expect that they would not be as much confined to small vertical limits, as 
animals : but this is not the fact; their vertical range, if any thing, is less than that of mol- 
