Kimball — Siderite-basins of Hudson River Epoch. 15 7 



still preserved are in monoclinal dips to the east, and expire 

 under cover by attenuation. The broken or basset edges of the 

 ore-beds outcrop on the west slope and near the crest of a line 

 of prominent ridges, the first from the river, known as Mt. 

 Thomas, Cedar Hill and Plass Hill. The several basins have 

 thus been elevated into what previous to the sculpturing of 

 the immediate valley of the Hudson was a range of anticlinals 

 of which the remnants are still prominent landmarks. The 

 thickness of the ore-beds along the escarpment formed by this 

 outcrop varies according to the relations of the line of obliter- 

 ative erosion with different sections of lenticular bodies, as 

 well as to the expanse and depth of the original basins of 

 deposit. For all are distinctly lenticular in shape when 

 referred to their uneroded condition, and considered as rem- 

 nants of a whole which corresponded each to a cast of its 

 original basin. The disrupted edge of the ore-body of the 

 second basin (in order from south to north), in the crest of 

 Mt. Thomas is near 600 feet above the river and about 44 feet 

 in maximum thickness. Similar escarpments in the third or 

 Cedar Hill basin expose a thickness of 30 feet of ore, and in 

 the fourth or Plass Hill basin, 8 feet. The thickness of the 

 filling of each basin is proportional to its original expanse 

 approximately determined by a horizontal projection of the 

 anticlinal arch. The first, a stnall basin, is wholly submerged 

 though at a shallow depth. This has been partially exhibited 

 by excavations along its upturned edge, and its extent deter- 

 mined by diamond drill. Though bodily lifted upon the 

 flanks of an anticlinal, mostly eroded, this ore-body preserves 

 the configuration of a basin and, like all the basins, is faulted 

 and thrown in minor degree. It has entirely escaped erosion. 

 The floor of all the basins is gray argillyte weathering into 

 brown shales, of Hudson River age, according to the earlier 

 views of Prof. W. W. Mather and Prof. James Hall, sustained 

 by Mr. T. Nelson Dale's discovery of fossils of that group in 

 the same formation near Poughkeepsie.* This is the lower- 

 most formation brought to the surface in this part of the 

 Hudson River valley, and forms the beds of that river and of 

 the lower parts of its eastern affluents in Columbia County. 

 It was penetrated in boring No. 1 to a depth of 662 feet, from 

 an elevation 222 feet above the river, the total depth of this 

 boring being 987 feet. Overlying conformably the Hudson 

 River shales is a thin belt of limestone intercalated with argil- 

 laceous shales, and continuous with calcareous grits, forming 

 the roof of the ore-basins. Numerous sections of this within 

 the compass of the first and second basins have been obtained 

 by use of the diamond drill. 



* This Journal, xvii, 1879, p. 377. 



