POSITION AND THICKNESS OF THE ROCKS 409 



The thickness of the Ordovician limestones, measured from the lowest 

 exposure in the section to the base of the Utica shale, is 5,752 feet accord- 

 ing to the author's measurements. The total thickness of the shales 

 between the top of the Trenton and the base of the Oneida is 1,000 feet. 

 Though there is a decided lithological and faunal break between the 

 Utica and the Lorraine shales, it is difficult in this case to define the exact 

 boundaries of the two formations. The interval of 350 feet between the 

 known Utica and Lorraine is occupied by a shaly formation apparently 

 devoid of fossils. At present, therefore, it is impracticable to classify this 

 intervening formation. Lithologically the doubtful formation is more 

 closely allied to the Utica than to the Lorraine. If the known range of 

 Triarthrus hecki is used as the determining factor in estimating the thick- 

 ness of the Utica, it does not exceed a thickness of 300 feet. If, how- 

 ever, the middle non-fossiliferous formation be added to it on account of 

 its lithological resemblances, then the Utica reaches a thickness of G50 

 feet. On this basis of division the Lorraine shale is 350 feet thick. 



Character of the Rocks 



in general 



Extended descriptions of the rocks in the Bellefonte region have been 

 given by various investigators connected with the different Pennsylvania 

 surveys. 



Rogers speaks of the enormous thickness of the Auroral magnesian 

 limestone. He divides the calcareous rocks of the valley into two gen- 

 eral classes, which he describes in the following terms : 



*' One of these is a rock of rather dull gray aspect and a crystalline or granular 

 structure ; this variety is decidedly ferruginous. The other formation is a remark- 

 ably smooth and fine grained rock of very pale blue color. It is very uniform in 

 texture and consists apparently of excessively comminuted particles. It is highly 

 magnesian, the weathered surface being coated with a white crust. The total 

 thickness of the formation visible in Nittany valley in the vicinity of Bellefonte 

 considerably exceeds 5,000 feet." * 



Rogers again divided the limestones into two groups based on the 

 presence or absence of fossils. The lower or unfossiliferous group is 

 4,800 feet thick ; the upper or fossiliferous horizon is 600 feet thick. He 

 regarded the upper group as the equivalent of the Birdseye and Black 

 River formations of New York. He assigned a thickness of 150 feet to 

 the Birdseye and 450 feet to the Black River. Rogers evidently inferred 

 the presence of Birdseye limestone on lithological grounds, since he cites 

 no list of distinctive Birdseye fossils. 



*H. D. Rogers: Geol. of Pennsylvania, vol. i, part i, p. 470. 



