JEFFERSON COUNTY. 



387 



Trenton Limestone. 



In this county, this rock furnishes two quite distinct varieties : a compact black mass, and 

 a grey crystalline one. The former is sometimes even-bedded, witli masses of slate inter- 

 vening ; at others, it is uneven, irregular and lumpy, with the same quantity of slate as before 

 investing the irregular masses. The grey variety occupies here the highest position, though 

 it is by no means constant ; or at least the lower dark colored and compact beds alternate 

 in some locahties with the grey crystalline mass, so that the latter is as often beneath as 

 tlie former. This fact alone, therefore, prevents the division of this rock into two species ; 

 but if this were not the case, and if the grey mass occupied uniformly the superior position as 

 in Jefferson, yet as the fossils in both masses are mostly the same, it would still be inexpe- 

 dient to divide it and form two distinct rocks. 



The trcnton limestone rises from the Black river in a series of terraces to the south, though 

 they are not uninterrupted. The first is in the form of a steep bluff from the river to the 

 platform upon which the village of Watertown is built. This extends south half a mile, when 

 it descends nearly to the level of the river ; from this valley the rise is regular for two or three 

 terraces, where it again descends into a deep valley whose bottom is not much higher than 

 the river at the village. It then rises in rather steeper and less regular terraces, and forms 

 the high range of limestone tlu-ee and a half or four miles south of the village. 



The structure and form of the terraces which rise and descend in succession as has been 

 described, were not produced by uplifts, or by derangements in the strata in consequence of 

 disturbances, but solely by the action of currents bearing along drift. The minor valleys thus 

 cut out of the trenton extend in the direction of the Black river, and range more to the east 

 than the valleys which have been described in the preceding pages. 



For a farther explanation of this rock, and the changes which it has undergone, I have 

 introduced the following section, which extends south from Watertown four miles, over the 

 terraces herein described : 



98. 



Section from Watertown four miles south. 



1. Birdseye forming the channel of the Black river. 



2. Isle La Molte marble. 



3. 3. Trenton limestone. 



4. Grey crystalline variety. 

 A. First valley of denudation. 



B. Second valley of denuda- 



tion, each of which extends northeiust and southwest. 



The terrace (No. 4) is covered with a thick mass of debris, which conceals the rock except 

 in a few points. Boulders of all kinds have lodged upon this hill, while the valleys below 

 are filled with sand. 



