TRENTON LIMESTONE. 113 



However this may be, it is very clear tliey are both to be considered as one mass or rock, 

 inasmuch as their fossils are identicaL 



In taking a retrospective view of the hmcslone of the Champlain group, we may see llie 

 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 ma}' 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 countv, 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 irregvdar 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 all 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 bj' 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 sufl'ered 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 g:-ey variety, when even-bedded, is a valuable mate- 



Geol. 2d Dist. 15 



