60 W. S. Bayley — Rocks of Pigeon Point, Minnesota. 



After an inspection of these figures there can be no rea- 

 sonable doubt that the two rocks from Pigeon Point are 

 parts of the same mass. The very slight differences in 

 amount noted in the case of the silica, alumina and potash 

 are not greater than are frequently found in different portions 

 of the same hand specimen of most rocks. The slight differ- 

 ence in specific gravity are what might be expected from a 

 study of the structure of the rocks. 



Unfortunately no complete analyses of Irving's Keweenawan 

 quartz-porphyries are given by that geologist, but a few sil- 

 ica determinations have been recorded, which are of interest 

 in showing the close agreement, in this respect, between these 

 rocks of undoubted eruptive origin and the Pigeon Point rock. 

 The percentages of silica in three Minnesota quartz-porphy- 

 ries* are respectively 71 "10, 73-87 and 76 - 83 ; thus differing 

 but slightly from the 74 per cent, of the Pigeon Point rock. 



In consideration of the large amount of sodium indicated in 

 analysis I, it was thought interesting to separate the feldspar 

 from one of the freshest of the red rocks and subject it to a 

 chemical examination. This was done in the usual way, and it 

 was found that the greater portion fell when the specific grav- 

 ity of the solution used was 2*577. This was analyzed by Mr. 

 Whitfield of the U. S. Geological Survey with the following 

 result: 



Si0 2 Alo0 3 Fe 2 3 CaO MgO K 2 Na 2 Ign 



65-00 18-22 2-64 1*06 0-06 4-18 8-40 -46 = 100-12 



When examined under the microscope the powder of this 

 mineral is seen to be free from quartz and quite homogeneous, 

 although slightly altered and filled with little plates of hema- 

 tite. Its optical constants could not be accurately determined, 

 but from the figures given above there can be but little doubt 

 that the feldspar is an anorthoclase.f 



If this be true the rock would fall into the quartz-kerato- 

 phyre group as defined by Posenbusch.^ Its microscopical 

 characteristics correspond to those of the quartz-keratophyres, 

 as described by Giimbel and Lossen, and the composition of its 

 feldspar is that of an anorthoclase. 



Many of the quartz-porphyries of the Keweenawan series, as 

 well as some of the augite -syenites will probably be found to 

 belong to this same class of rocks — a class which up to the pres- 

 ent time has not been known to have a representative on this 

 side of the Atlantic. 



One of the most interesting points in the study of the red 

 rocks of Pigeon Point, has reference to the origin of those 



* Copper-Bearing Rocks, pp. 108, 109, 100, 441. 



f Anorthoclase separated from a liparite of Pantelleria, has a composition 

 (according to Forstner, Zeitschr. f. Kryst., 1883, p. 125) as follows: SiO 2 = 66-06, 

 Al,O s 19-24, Fe 2 3 054, CaO 1-11, MgO 0-11, K 2 5-45, Na 2 7-63. 



\ Rosenbusch: Mikroskopische Physiographie, 1887, ii, pp. 434-442. 



