Cushietg — Potsdam Boundary. 23 



with white is opened and has a considerable local use. White sandstone 

 is exposed by the river at Nicholville, and near Fort Jackson much quarrying 

 is done in white and Luff sandstone. Westward from this line of outcrops no 

 Potsdam was seen in the township. 



Parishville. 



In this township the boundary at first bears to the north to enclose 

 the massive but low ridge of gneiss lying to the northward of Parish- 

 ville village. The exposures are excellent and show a well foliated gneiss of 

 intermediate composition, for the most part with a considerable bi-silicate 

 content which is usually hornblende. This is sometimes replaced by a 

 pleochroic augite. Biotite is also a prominent ingredient and a little muscovite 

 shows in some of the slides. The predominant feldspar is plagioelase, either 

 oligoclase or andesine, but orthoclase, microcline and quartz are present in 

 considerable amounts. 



Just to the westward of Parishville village, the Grenville series comes to, 

 and forms the boundary. Bocks which unquestionably belong to that series 

 are separated by ridges of gneiss of uncertain relationship, but these excepted, 

 the Grenville series remains at the boundary as far as the work was carried. 

 The unequal resistance to erosion presented by the various members of this 

 series, coupled with the fact that the general altitude of the country is much 

 below that to the eastward so that erosion does not go forward so rapidly, 

 combine to render the boundary very uncertain, both because of the exceed- 

 ingly erratic distribution of the Potsdam and because it rarely shows in 

 outcrop. 



These rocks will be described by Professor Smyth. It may be said in gen- 

 eral that the gneisses in this series differ from the older gneisses in being for 

 the most part much more finely and evenly granular and in having, in many of 

 the beds, abundant biotite as the only ferro-magnesian mineral. 



The Potsdam was found exposed in but two localities in the township, 

 both along the brook which empties into the river one mile west of Parishville. 

 The best exposures are about a mile up the brook and consist of coarse sand- 

 stone of medium induration, striped in white and flesh color, or white and buff. 

 The other outcrop is further north where the road from Parishville t<> 

 Parishville Centre crosses the brook, and the exposure is but meagre. These 

 rocks lie in a trough eroded in beds of very quartzose gneiss which oppose but 

 feeble resistance to degradation. Sufficient data were not obtainable to admit 

 of determining whether the Potsdam owes its present position to faulting or 



