384 
GEOLOGY OF THE FIRST DISTRICT. 
part of the county. Although the same rock forms the basis of a large part of the adjoining 
counties on the north, south and west, it does not extend unbroken into either of them.” “ On 
the east side, it terminates abruptly below the brow of the hill (mountain) which bounds the 
valley of the Little Hoosick on the west.*” “ Wherever the greywacke rests immediately 
on the argillite, it is the slaty variety, and takes on the form and position of the argillite.”* 
“Though greywacke slate is generally of a g4py color, it is found of a sky blue colorJn 
many places, and sometimes brick red. There is a limited locality of the latter color on the 
line between Troy and Brunswick, on the Sandlake road. On ascending the hills, we find 
the fissile slaty varieties of greywacke giving place to the more compact, and the color be¬ 
comes more blue and greenish.” “ Farther east, in the interior of the county, as in Grafton and 
Sandlake, we find the beautiful green and blue varieties very abundant; but on ascending the 
higher hills of those towns, the only rock in place that presents itself, is the coarse rubblestone 
variety of greywacke.” “ This rock has formed a subject of some discussion. It puts on 
so many appearances, that it has been a source of much confusion when studied in the state 
of hand specimens. There are numerous localities in the towns of Grafton and Sandlake, 
where this rock in a single, continuous, unbroken field, presents the green greywacke, the 
coarse granulated variety, the porphyritic variety which is speckled with felspar, and that 
variety which has the appearance of puddingstone. I have examined vast rocks of the com¬ 
pact, blue, and green greywacke, which embraced within their most solid parts large masses 
of an aggregate in all respects resembling the common puddingstone.”* 
It might be inferred from the above quotations, that the coarse greenish grit rock and con¬ 
glomerate, which sometimes approaches in character to porphyry and greenstone, was super¬ 
imposed on the slate rocks unconformahly. It may be so situated, but I have seen no evidences 
that would lead to that conclusion. 
The slate rocks of the Hudson-river group in Rensselaer county, and in fact in all the 
counties underlaid by the upturned rocks of the Champlain division, are very difiicult to dis¬ 
tinguish from those of the Utica slate (or black slate), except where the latter contains fossil 
remains (graptolites), or thin layers of limestone. The upper layers of the slate of the Hud¬ 
son-river group also contain graptolites, some of the species of which are different from those 
of the Utica slate. In consequence of the difficulty of distinguishing these two masses 
where they and the included mass of interstratified grits and slates are broken up and con¬ 
fused in their stratification, I have barely mentioned the slate ; but it ranges along the banks 
of the Hudson, from Fort-Edward by Troy, Greenbush to Castleton, and still farther south, 
and is frequently seen interstratified with the Utica slate and its thin bands of limestone. 
* Geological and Agricultural Survey of Rensselaer County, 1822, p. 22, 21. 
