194 Liexd.- General G. A. McMahon — Allanite in Granite. 



convex to the frout (as usual), sligMly thickened, and the anterior 

 covered border smooth and concave (preserved for about three lines 

 in antero-posterior length). The dorsal surface, from the anterior 

 margin to about ten lines back, is covered w^ith the characteristic 

 scale-like markings of the genus, composed as usual of semi-oval, 

 narrow, prominent ridges, the wide open part of the curve directed 

 forwards and the convexity backwards, averaging about ten in 

 longitudinal and about seven in transverse direction in six lines, and 

 rather more regularly equal than in P. bilobus. The entire antero- 

 posterior length of the segment is 2 inches 5 lines, the posterior 

 portion, of about 1 inch 9 lines, as usual in the genus, being free 

 of scale-like sculpturing. The entire breadth of segment I estimate 

 VFOuld have been about 6J inches. 



None of the Merostomatous Crustacea have been found before in 

 this country, and Mr. Spry has presented the type-specimen to the 

 National Museum. 



II. — On the Oocurrknoe of Allanite in the Hoknblende- 

 Gkanite of Lairg, Sutherlandshire. 



By Lieut. -General C. A. McMahon, F.R.S. 



IN the September number of the GEOLoaiCAL Magazine for 1898, 

 Mr. J. S. Flett, B.Sc, describes, at p. 388, the occurrence of 

 Allanite in the granite of Fell Hill, Cree Town, Kirkcudbrightshire, 

 and in a biotite gneiss in Sunday, one of the Orkney Islands. 



I have recently been re-examining a slice taken from a sample of 

 Heddle's ' Syenite ' collected by me at Lairg, Sutherlandshire, some 

 years ago, and I find it contains a very good crystal of allanite. 



The rock is a hornblende-granite, and the slice, when examined 

 under the microscope, is found to contain the following minerals ; 

 namely, orthoclase, plagioclase, quartz, biotite, hornblende, sphene, 

 apatite, zircon, chlorite, and allanite. 



The sphene is rather abundant and is idiomorphic in characteristic 

 forms. The chlorite is the product of the alteration of biotite. 



The allanite is in a lath-shaped, badly terminated, crystal, which 

 extinguishes, and has the major (-{-) axis, at 36° to the line of 

 elongation. The slice is therefore one parallel to the clinapinacoid. 

 It is of pale orange-yellow colour in transmitted light (my slice is 

 a very thin one), and is distinctly but feebly pleochroic. When 

 revolved between crossed nicols it shows, prior to extinction, 

 a decided zonal structure, which cannot be seen in ordinary trans- 

 mitted light. The crystal is traversed by irregular cracks, more 

 or less transverse to the direction of elongation ; but no regular 

 cleavage is to be seen. 



Good-sized crystals of allanite appear to be rare ; but crystals 

 and grains of mici-oscopic size are numerous in the rock, and 

 I succeeded in isolating many of them. The mineral in the Lairg 

 granite is brown-red in reflected light, and when the rock is pounded 

 up its peculiar colour marks it ofi^ from the other minerals and 

 enables the petrologist to pick it out from them with the aid of 



