ROCKS RESTING UPON A PORTION OF THE TACONIC SERIES. 87 



ni. OF THE ROCKS WHICH ARE KNOWN TO REST UPON A PORTION OF 



THE TACONIC SERIES. 



For a complete elucidation of the relations of the Tacoiiic rocks, it is necessary that I 

 should speak again of those which lie in a belt of country in the middle portion of Rensse- 

 laer county, east of the Hudson river. This will give me an opportunity of adding to the 

 illustrations of one or two interesting points in Washington county. For this puipose, I 

 shall speak of the succession of rocks upon the Western Railway, between Chatham four- 

 corners and Greenbush. At Chatham four-corners an unequivocal slate of the Taconic 

 system makes it appearance, with the regular southeast dip and northeast and southwest 

 trend. The angle of dip is high, and the rock presents that peculiar short wrinkled con- 

 dition common to it : it contains also the milky quartz in seams. At the village, it appears 

 to dip down steeply, or falls off rapidly from a modestly elevated ridge which skirts it on 

 the east; and in consequence of tliis westerly plunge of the wliole mass, the rock wholly 

 disappears beneath a thick deposit of moderately coarse drift. Proceeding west from 

 Chatham four-corners, upon a tolerable level waj', no rock is seen in place for a mile, 

 when there occurs a dark and greenish shale more or less flinty. This shale proves to 

 belong to the Hudson river series ; at least it contains an abundance of several species of 

 Graptolites, known elsewhere in the series ; it is, however, intermixed with red and 

 chocolate-colored slates. These together continue half a mile, or make their appearance 

 a few times in this distance; finally the thick-bedded grey sandstone appears interlaminated, 

 as it so frequently is elsewhere, with fine blue slate without fossils. A succession of this 

 kind continues five miles, following the railroad route ; that is, there arc frequent uplifts 

 of the shale, and thick-bedded sandstone with its interlaminated slate. Of the character 

 of this rock, there is not the least doubt : it is that which forms the northern slope of the 

 Helderberg, and is elsewhere known as the Hudson river series. Now, although the dip 

 is in a measure in the direction of the Taconic slate, still all sound observations show that 

 it is an unconformable rock. Proceeding to Chatham centre, the succession thus far is 

 clear ; but after leaving the latter place, we soon come upon very thin-bedded limestone 

 slate, with silico-calcareous layers and slate interlaminated, but the whole thin, and 

 altogether similar to the Taconic slates. Whether these thin beds are really as I have 

 supposed, is a matter of little consequence, since we soon come upon one in this direction 

 of which there is no question that it is the Taconic slate. This slate continues about seven 

 or eight miles, appearing only occasionally above the drift. In about five miles of the 

 Depot at Greenbush, the Hudson river group succeeds ; the Taconic slate again appearing 

 in the form of the shales, and in that of the thick-bedded sandstone mentioned above. 

 Approaching the river, the beds are more disturbed, and of a more equivocal character. 

 They are not only bent, but crushed ; and a bed of siliceous limestone six or eight inches 

 thick is not only broken into short pieces, but it is distributed throughout a mass of broken 



