Williams — Contact-metamorphism near PeeJcshill. 263 



biotite, magnetite, and- margarite, which latter mineral will be 

 particularly described under the next group. 



Group 4 comprises inclusions which are largely composed of 

 a micaceous mineral, bearing a close resemblance to muscovite. 

 It forms a confused aggregate of flakes with no regularity in 

 their arrangement, and is associated with varying amounts of 

 green mica, tourmaline, magnetite, epidote, staurolite, etc. 

 Specimen No. 32, taken from an inclusion in the mica-diorite 

 near the head of the road leading up the hill at Cruger's 

 Point (s on the map), is almost wholly composed of this 

 mineral. Macroscopically it bears every evidence of being 

 muscovite, but a careful examination shows that it possesses 

 several characteristics which are foreign to this species. 



Under the microscope the lath-shaped cross-sections at once 

 appear different from muscovite by their high refractive index, 

 which throws their boundaries and cleavage lines into high re- 

 lief. Moreover they show between crossed nicols a distinctly 

 inclined extinction, varying, when measured against the cleav- 

 age, from 6° to 10°. There is also a very frequent twinning 

 parallel to the basal pinacoid. The interference colors in po- 

 larized light are as brilliant as those of muscovite, but the opti- 

 cal angle is far too large for that species. Measured in the air 

 it gave a value of 114^°, while the limit for muscovite is 70°. 

 The asymmetric position of the interference figure when exam- 

 ined in a cleavage flake also indicates, that the bisectrix is con- 

 siderably oblique to a normal to the basal plane. The charac- 

 ter of the double refraction is, as in the case of muscovite, 

 negative. 



The optical properties here enumerated are seen to be inter- 

 mediate between those of muscovite and the brittle micas or 

 chloritoids. The same thing is true of the hardness (3'5-4) 

 and specific gravity (3*1) of the mineral here under considera- 

 tion. All of these physical characters, however, agree accu- 

 rately with those given by Des Cloizeaux* and Tschermakf for 

 the lime mica, margarite, (=Perlglimmer Germ., Emerylite 

 Smith), which is well known to occupy a position between true 

 mica and chloritoid. This conclusion is fully substantiated by 

 the following analysis (I), made by Mr. T. M. Chatard, of the 

 U. S. Geological Survey at Washington. The rather small 

 amount of material for this analysis (0*4212 gr.), was obtained 

 from specimen No. 32 by as complete a separation as possible 

 with the Thoulet solution. A very small proportion of black 

 tourmaline still remained with the powder analyzed, which may 

 in a measure account for the rather high percentage of iron. 



* Manuel de Mineralogie, i, p. 501, 1862. 



f Die Glimmergruppe : Zeitschr. fiir Krystallogr., ii, p. 48, 1877. 



