9 8 



Lower Devonian, one of Bohemia and the other of New- 

 York. 



These beds are not distinctly developed elsewhere in 

 the region and they appear to represent a Devonian stage 

 earlier than that of the Perce Rock. 



The Rock Wall between the North and South Beaches. — 

 Just at the steamboat wharf on the North Beach 

 recent excavations, now covered, exposed a grey, steeply 

 inclined shale carrying Dipterus. No other fossil 

 has yet been determined from this shale which is 

 regarded as belonging to a Devonian stage beneath that 

 of the Perce massive. This shale is apparently faulted 

 against the Silurian at the south and limited at the north 

 by the fault lines of the beach. Following the shore 

 southward the first outcrop is of the erect grey limestones 

 and shales of Mount Joli. The rock exposures begin with 

 the reefs exposed at low water to about 400 feet from 

 the shore and the Mount Joli cliff as a whole has a sea front 

 of about 700 feet (211 m.) and the same dip as the Perce 

 Rock strata. This would give the formation here an 

 approximate total of 1,100 feet (335 m.). There is little 

 change throughout in its lithic characters, but there is clear 

 evidence of a displacement within the mass which gives a 

 geological meaning to the division of the massive into a 

 north flank and a south flank. The beds of the north flank 

 afford admirable exhibitions of jointing and ripple marks 

 and in both flanks fossils are to be found in thin beds with 

 barren intervals. In the north flank are the corals Dun- 

 canella, Zaphrentis, Streptelasma and Pleurodictyum, the 

 graptolite Monograptus cf. clintonensis, the brachiopods 

 Dalmanella, Leptcena {rhomboidalis) , Stropheodonta, Spi- 

 rifer (cf. niagarensis, modestus) and an uncertain Phacops: 

 all of which indicate a Silurian stage. 



In the south flank of Mount Joli are the trilobites Ampyx, 

 Tretaspis, Calymmene, Trinucleus, Pterygometopus, Pty- 

 chopyge and Illanus, the brachipods Dalmanella, Rafines- 

 quina, Strophomena, Parastrophia, Zygospira, the assem- 

 blage indicating a middle or later Lower Ordovician stage. 

 These beds seem to extend across the mountain and just 

 touch the other shore near the wharf house. 



This exposure ends abruptly at the south in a short 

 beach covering a fault, beyond which is Cap-du-Canon, 

 a mass of erect, dark argillaceous and calcareous slates, 

 much crumpled and glazed. The general inclination 



