22 Geology and Mineralogy of a part of Massachusetts, &c. 
tially from that on the west of the transition limestone, 
there is reason to conclude that this stratum of argillite ex- 
tends to the Hudson. [If this be true, the stratum of gray 
wacke actually lies over and upon the argillite.* The only 
reason for doubt is, that the inclination of the gray wacke 
and the occasional interposition of narrow strata of argillite, 
might lead to the conclusion that the argillite and gray 
wacke alternate with each other. I have often thought, 
when examining these rocks, that this is the fact. Future 
observations, especially upon the situation of the gray 
wacke on both sides of the river, will doubtless determine 
which is the fact. 
From examination of the argillite in Columbia Co. 
there cannot be a doubt that the argillite of the Hudson 
extends a considerable distance to the east,—the fracture, 
variety of color, seams, facility of disintegration, and-gen- 
eral appearance, all concurring to prove the rock in diffe- 
rent places to be the same stratum. 
Very few petrifactions have been found in this argillite, 
at least in this section. Perhaps the only one is the ortho- 
ceratite, mentioned in the Geol. of the northern states, 
page 167. This rock, however, contains beds of siliceous 
slate, which affords petrifactions. The siliceous slate near 
the city Hudson, contains pectenites and terebratulites, and 
is found in extensive beds. 
The rapidity with which this argillite is disintegrated on 
proper exposure has already been mentioned in this Jour- 
nal, Vol. Il. p. 248. 
Glazed Slate. Eaton. 
This curious variety of transition argillite is found at 
Troy, Lansingburgh, &c. and would probably be found gen- 
erally along the Hudson by passing to a certain depth 
in the common argillite. The structure is distinctly 
slaty, but the lamina have a much finer and more com- 
pact texture than the common argillite above. It is full 
of irregular seams, or the lamina are of very irregular form, 
so that it easily breaks into.a multitude of lenticular, splinte- 
* See note, page 21. 
+See his Geol. Survey of the County of Albany, and also of the County 
of Rensselaer. 
