Pirsson and Washington — Geology of Red Hill, N. II 271 



tion ; the direction of a and its angle, as actually measured 

 with the vertical axis c given by the twinning line and lamellas, 

 are shown in the figure. These, with the interfacial angle of 

 101 on 100, are the characteristic properties of wohlerite and 

 positively identify it.* The manner in which the hyperbolas 

 around c pass out of the field indicates a large obtuse angle. 

 JNearly all the crystals are twinned, some like those in the 

 figure, others much more irregularly. Some of the crystals 

 show the negative unit orthodome 101, and some a positive 

 orthodome, of which the development and measurements were 

 poor, and hence no symbol is assigned : it may be 101. In 

 general the crystals are colorless, but one or two instances 

 were noticed where they showed the characteristic pleochroism 

 in tones of yellow; such sections showed higher birefringence 

 and were therefore cut near 100; this is another characteristic 

 of the mineral. The crystals were broken by stray cracks 

 which could not be oriented as definite cleavages. Their gen- 

 eral form is thick tabular on 100. They are all fresh, only in 

 one instance was the mineral noticed altering into a some- 

 what brown, earthy substance. The only occurrence of this 

 mineral other than in the pegmatite dikes of South Norway 

 that I have been able to find mentioned in the literature is 

 from inclusions iri phonolite at Pertuis, France, by Lacroix in 

 his Mineralogie de France, though another doubtful one is 

 recorded by Breithaupt and Cotta at Ditro. 



Mode. — The determination of the actual mineral composi- 

 tion of a coarse-grained rock, such as this, by the Rosiwal 

 method presents some difficulties when it is attempted under 

 the microscope. The following process was adopted in this 

 case. The total area of a very large thin section exposing 

 about 4: by 3*5 cm of rock surface was carefully determined and 

 found to contain 1350 mm2 . In this the total area of all of the 

 ferromagnesian minerals w T as computed and found to be 81 mmQ , 

 which equals 6 per cent. If we reckon these areas as equiva- 

 lent to corresponding volumes and assign a specific gravity of 

 3*3 to the hornblende and 2*6 to the feldspar, we have felds- 

 pathic minerals = 92*7 per cent, ferromagnesian 7'3 per cent. 

 If in the norm given beyond we allot the anorthite molecule 

 to the femic minerals, since actually the lime silicate is in the 



* The different handbooks are not in agreement on the position of the 

 axial bisectrices of this mineral. Dana. Hintze and Levy and Lacroix give 

 c = b, Kosenbusch and Iddings give a = b. This is apparently due to 

 Brogger (Zeitschr. fiir Kryst., vol. xvi, p. 359, 1890), who states, following Des 

 Cloiseaux, the former but in the figure, plate xviii, gives the latter orienta- 

 tion, an error probably due to the engraver confusing a and c in German 

 script. The first named authors have followed the text, the others appar- 

 ently the figure, which last is undoubtedly wrong, as may also be seen under 

 hiortdahlite, p. 378. 



