part 2] GEOLOOY OF THE MELDON VALLETS. 103 



As differentiating the rock from the general mass of the 

 Dartmoor granite, analysis reveals two features only. In the 

 first place, the iron, determined as peroxide, is but 025 per cent., 

 whereas the normal granite yields both ferric and ferrous oxide, 

 which (if determined as the former oxide) would usually reach 

 T90 to 2*00 per cent. In the second place, soda is distinctly the 

 dominant alkali ; while in the normal granite there is more potash 

 than soda, but the excess of the former is not great. 



Four reliable determinations of the alkalis in the Meldon aplite 

 are available. The mean of the four gives total alkalies as 8*70 

 per cent. The lowest record is 8*13, the highest is 9*20, and the 

 intermediates are both 8*75 per cent. The relative proportions of 

 potash and soda are, perhaps, not quite as satisfactorily determined 

 in two of these analyses. The most probable mean yalues would 

 appear to be potash = 3*40 per cent., soda = 530 per cent. 



The specific gravity of the rock is 266. At or about a 

 temperature of 950° C. the stone softens, and at 1070° C. it flows, 

 forming a white enamel-like substance, with sometimes a tinge 

 of cream ; at higher temperatures, not precisely determined, it 

 melts to a pale-green glass. 



Heated to well below redness the rock phosphoresces, emitting 

 white light. Tested in 3-inch cubes the average crushing strain of 

 the Meldon aplite is 24,000 lbs. to the square inch, or 1-549 tons 

 to the square foot, as compared with Penryn Granite (Cornwall) 

 1060 tons, and Hill of Fare (Aberdeen) 1360 tons, both rocks 

 noted for their strength : this is distinctly high. 



The general colour is a xevy pale blue-grey, almost white, but 

 there are local variations, for the greater part very restricted in their 

 occurrence. The most usual deviation is to a pale azure — this is 

 found near junctions with the shale, and arises from the free 

 development of microscopic blue apatite. A much less frequent 

 shade is rose-pink, caused by partial alteration of the felspar. 

 Veins stained blue by apatite, or purple by fluorspar, are also found, 

 and occasional veins of lilac or rosy-lilac colour, which will be 

 described later. Joint-faces may be emerald-green when lined 

 with tourmaline, rose-pink when coloured by montmorillonite. 

 purple from the development of fluorspar, indigo-blue when apatite 

 is present, or of a silvery lustre when coated with mica. 



The body of the dyke is very tine-grained, with no porphyritic 

 constituents; but it is traversed by veins of coarser material, in 

 which the normal minerals, with the exception of topaz, are 

 repeated in larger form, some felspars measuring over an inch in 

 length. These veins are mostly narrow, a few inches being the 

 limit of width ; occasionally they pass out from the aplite into 

 the shales. 



The minerals of the aplite are:- Felspar, quartz, mica, tourma- 

 line, topaz, apatite, fluorspar, montmorillonite, and axinite. 



The texture is microgranitie | PI. VII, fig. 12). C. A. McMahon 

 notes (4, p. 388), that a striking feature of his slides is that the 

 prisms of felspar are sometimes bent and in some cases broken ; 



