18 S& W. Ford—Great Fault in Rensselaer County. 
zone. This group is represented upon the map by the ob- 
liquely lined area. A brief description of these older rocks, by 
the writer, is contained in the July number of this Journal, for 
1884, and since that time I have been able to increase but little 
the already published list of species found in them. Some 
distance south of the most southerly reach of the Lorraine, 
the Primodial strata begin to run inland; and, continuing 
this, finally sweep past the point ¢ in a broad, northeasterly- 
bearing curve. In one favored locality, the lower slates of this 
group are found reposing fairly upon those of the Lorraine; 
and upon the phenomena presented at this point, I propose 
now to speak. 
The locality referred to is situated nearly due south of Mr. 
McCabe’s residence, and, by pacing, 210 yards distant from it. 
It may be easily found by going directly across the low ground 
in front of the house to the ledge c, and then taking the by- 
road southward along the edge of the hill. ‘I'bis road is only a 
few rods distant from the Hudson River railroad track. A 
small spring issues from the slates directly on the line of con- 
tact of the two groups; and although, at first sight, the older 
slates appear to conform perfectly with the newer, a. closer ex- 
_ amination shows that their cut edges, owing to the curve of the 
beds, really make a small angle with them. Moreover, the 
slates of the two groups are lithologically different, those of 
the Lorraine being, for the most part, black and carbonaceous, 
and some of them graptolitic up to within a few inches of the line 
of the overlap ; while those of the Primordial are unctuous and 
highly micaceous, or, as Professor Dana informs me, may be 
true hydromica schists. There can be no doubt of the existence 
here of a great physical break, the course of which is indicated 
upon the map, by the continuous line bounding the Primor- 
dial area upon its western side. 
In order to make the leading structural features of the region 
under consideration still better understood, I give below two 
sections, one of them along the line a a of the map and the 
other along the line 6b. In the first of these (fig. 1), the’ pre- 
