thenton limestone. 
113 
However this may be, it is very clear they are both to be considered as one mass or rock, 
inasmuch as their fossils are identical. 
In taking a retrospective view of the limestone of the Champlain group, we may see the 
very curious and interesting changes which have occurred from one period to another, in the 
nature of the materials which enter into the composition of these rocks. The lowest of the 
limestones is very siliceous : this earth, together with an uncrystallizable matter, gradually 
disappears, when the limestone of the birdseye becomes perfectly pure ; though the peculiar 
state and condition of the particles is such as do not favor crystallization, excepting in those 
points where organic matter is inclosed. After the deposition of the latter rock, argillaceous 
matter first appears, which gradually increases as the deposition of the Trenton limestone 
progresses, till finally the calcareous . matter disappears, being mostly replaced by a dark- 
colored clay or mud, giving origin to a great thickness of slate rock, shales, etc. We shall 
find that these are succeeded by a shaly sandstone, and finally by nearly a pure siliceous 
deposite. 
Returning to the consideration of the Trenton limestone, the reader will perceive from the 
remarks already made, that it may be divided lithologically into two masses : the close-grained 
black variety, and the grey crystalline one. This division, however, is not so important as 
might appear at the. first view of the subject.- It is true, that in Jefferson county, the two 
varieties occupy distinct geological positions; but, examined in a wider field, this collocation, 
does not appear to be uniform ; in fact, the relative position is sometimes reversed, as at or 
near Montreal in Lower Canada. The black shaly mass differs a good deal in its appear¬ 
ance : at one place, it is a very even thin-bedded rock, with regular layers of shale interven¬ 
ing ; at another, it is lumpy, as if the calcareous matter had accumulated rapidly, and taken 
immediately a concretionary movement, by which irregular oval masses were formed, around 
which the argillaceous matter accumulated in irregular planes, or planes corresponding to the 
uneven surfaces which would necessarily be formed under such a condition of things. This 
destroys alf regularity, therefore, in those minor beds, which, by'exposure to frost and other 
agents, are rapidly broken up, and the lumps of limestone become coarse, and gradually ac¬ 
cumulate about the beds or at the foot of declivities.. 
The vignette at the head of this section, exhibits a view of the condition as well as the re¬ 
lations of the rock at Watertown, on the banks of Black river. The lower part exhibits the 
birdseye, and the seven-foot tier, as it is called by quarry men; the upper part, a portion of 
the Trenton, the lower layers of which are much-broken, while the highest part in the draw¬ 
ing has suffered much'less in this way. The process of decay gradually goes on, and the 
higher layers are finally undermined, when they fall down and assist in forming the talus 
below. 
The Trenton limestone, for causes which will be recognized in these facts, is rarely a good 
building stone, and still less suitable for marbles, with the exception of the grey crystalline 
mass already adverted to. The former is shaly, and liable to split and break by the action of 
frost; or it is in lumpy masses, still more unsuitable for any purpose except for stone fences, 
or the coarsest kinds of structures. The grey variety, when even-bedded, is a valuable mate- 
Geol. 2d Hist. " ' 15 
