DISCRIMINATION OF CAMBRIAN AND SILURIAN LIMESTONES. 837 



by the quartzite series and disappearing to the north, lies in a trough of the 

 quartzite. 



The whole series has an easterly dip, but the apparent great thickness of 

 the Cambrian limestone — 3,000 feet, if the strata lie in a simple overturned 

 synclinal — seems improbable, especially as it is near the shallow ending of 

 a trough. Kepeated folding of a thinner series may have produced this 

 structure. 



AVhatever be the true thickness of the limestone, it cannot be represented 

 in the Centre Rutland belt, since the fossils make this Lower Silurian ; 

 hence, either (1) the Pine hill series is younger than the Cambrian lime- 

 stone (underlying it by inversion) and there is a continuous inverted series 

 from the eastern Olenellus quartzite to the Lower Silurian limestone ; or (2) 

 the Pine hill series is Cambrian, underlying the Rutland limestone as an 

 anticlinal overturned toward the west, and the great thickness of limestone 

 and quartzite is absent on the western side or represented by the Pine hill 

 schist only, through sudden change of deposition ; or, more probably, (3) that 

 a great thrust plane exists on the western side of Pine hill, by which the 

 Cambrian is made to lie on the Lower Silurian limestone. 



The first explanation seems impossible on account of the trough structure 

 of the Cambrian limestone and the perfect lithological identity of the Pine 

 hill series with the eastern quartzite series. The second, while not impossible 

 in this region, in which the Cambrian beds are liable to sudden great changes 

 in thickness and lithological character, is certainly improbable ; and the 

 third therefore remains as a working hypothesis, to be established by further 

 discoveries of fossils and observation of the actual thrust plane. The writer 

 is inclined to place this on the western face of Pine hill, at the junction be- 

 tween the limestone and schist band, where indications, but not decisive 

 proof, of such a thrust plane have been observed. 



No statement is here made as to the age of the schists in the next ridge 

 westward or in the Taconic range, nor as to the true structure qf this part 

 of the section. 



