90 NEW YORK STATE MUSEUM 



the grit at the base were it not for the fact tliat it is not always 

 in contact with the Georgian. But this fact, since we now know 

 that the Georgian and the Ordovicic are in many, or all, places 

 separated by an overthrust plane, is no longer of decisive value. 

 On a priori ground, since there is an important unconformity in 

 the slate region between the Lower Cambric Georgian rocks and 

 the overlying Ordovicic beds, marking a long period of emergence 

 and erosion, we should expect the Ordovicic series to begin with 

 the coarse grits, these being followed by the fine siliceous muds 

 that produced the white beds, and the latter again by the argilla- 

 ceous muds that become the dark graptolite shales. This succession 

 agrees with the Willard mountain section and appears to us the 

 true one. 



Another question which can not be satisfactorily answered is 

 that of the thickness of the Normanskill formation and of its 

 divisions in this region. Dale, in the above-mentioned ^table, 

 assigns the '' Hudson shales " a thickness of 50-]- feet ; the " Hud- 

 son white beds " 400 feet or less and to the '' Hudson grits " 500+ 

 feet; and in a later paper (1904, page 37) the "Hudson forma- 

 tion " of Rensselaer county (including the Snake Hill beds and 

 colored shales) is estimated at 1200 to 2500? feet. A former esti- 

 mate for the Hudson formation on the east side of the Hudson 

 river by Walcott (1890, page 346) had been 5000 feet. This, as 

 well as Ashburner's estimate of 3500 feet for the Altamont well, 

 are considered by Dale as too high, who holds that " in a region 

 of such moderate relief a mass of beds 2500 feet thick,^ thrown 

 into small, close and mostly overturned folds, would account for 

 such a rock surface as that depicted in that portion of the map 

 which lies west of the Taconic range." While we agree with Dale 

 in this latter view, we yet consider his estimates as giving the 

 minimal estimates, rather than the maximal ones, for where the 

 succession of faunules permitted the exclusion of the repetition of 

 beds as a factor in increasing the apparent thickness of the forma- 

 tions, considerably greater thicknesses were obtained by the writer. 

 In the case of the Deep Kill graptolite shales of Beekmantown age, 

 for instance, the faunal zones indicate a thickness of the formation 

 of from 200 to 300 feet, while Dale observed not more than 50 

 feet of this formation in any one place. It is true that Ashburner's 

 measurement of 3500 feet in the Altamont well is not applicable to 

 this shale region, because the shales at Altamont belong in another 



1 Dale's estimate of the combined thickness of the Lower Cambric and 

 Lower Siluric in the slate belt. 





