NOKTH-WEST REGION Or CUAKNWOOD FOHEST. 83 



microscopic structure between the Sbarpley and Peldar rocks are 

 not conspicuous. In the former, the dusty opacite is, as a rule, 

 only very rarely replaced by the green silicate already mentioned, 

 and is less regularly aggregated ; so that the rock, under the 

 microscope, has a more streaky or blotchy aspect instead of the 

 peculiar spotted or speckled look of the other. Still this may be 

 seen occasionally. 



Our old specimens have been repeatedly studied, and some new 

 ones examined, with the result that we feel more confidence in 

 attributing the occasional indications of a fragmental structure, 

 which the rock exhibits, to mechanical movements subsequent to its 

 consolidation — that is, to the cause which has produced its schis- 

 tosity ; and we find, in its general structure, increased resemblances 

 to a glassy lava, which has been subsequently devitrified. Traces of 

 pyroxenic minerals are rarer in the porphyroid of Sharpley than in 

 tliat of Peldar Tor. But in some slides, brown, almost opaque, 

 rather roughly shaped belonites, about "OOo" long, are fairly 

 common. These occasionally become almost translucent and colour- 

 less, perhaps owing to the conversion into chalybite of the colouring 

 limouite. They are feebly anisotropic, and may possibly indicate 

 the former presence of a pyroxene rich in iron. Larger grains of 

 an iron oxide, somewhat decomposed, occur occasionally, and the 

 granular blackened mineral, described in the Peldar-Tor rock, 

 is here much more rarely seen. 



The order of the phenomena in both rocks seems to have been 

 the same, and may be summarized as follows : — 



(1). Formation of quartzes, felspars (with a slight tendency to 

 occur in groups of two or three), iron oxides, and other minerals 

 ((?. //,, pyroxenic). 



(2). Partial corrosion of the quartzes and felspars, with occasional 

 fracture of the former (at least). 



(o). Consolidation of the matrix, segregation of opacite, forma- 

 tion of minor structures. 



(4). Production of a rude cleavage ; principal cracking of the 

 larger included minerals. 



(5). Devitrification, &c. — Order of (4) and (5) uncertain ; the 

 latter doubtless a very })rolonged process. 



An analysis of the Sharpley rock was made by Mr. Berry in 1882*, 

 and a partial one is given in our last paper f- The large amount 

 of Na.,0 compared with Kfi in the former analysis appeared strange, 

 for microscopic examination had indicated that a fair amount of the 

 porphyritic felspar was orthoclase. At our request. Miss Aston 

 kindly undertook to determine the amount of these constituents in 

 another specimen ; this gave Xa.,0 = 2*43 and K„0=2-18, which we 

 believe better expresses the normal composition. The percentage 

 of SiOj was high, about 77*8 J ; Mr. Berry obtaining 67*6, the other 

 observer 68*05. Probably ihe fragment recently analysed contained 



* Quart. Journ. Geol. Soe. vol. xxxviii. (1882) p. 199. 



t Ibid. vol. xxxvi. (1880) p. 342. 



J One analysis gave SiO.j==77"70 ; another, 77"88. 



