Vol. 51.] GRANOPHYRE AND THE GRAISTSGILL GREISEN. 129 



these radiate aggregates is determined chiefly by the minute radially- 

 grouped fibres of felspar which obscure the associated quartz. 



From the micrographic the transition is unbroken to what may 

 be called the ' cryptographic ' structure, in which the individual 

 elements of the intergrowth are no longer to be distinguished, 

 Here presumably the constituent sectors are still more attenuated, 

 aud the radial arrangement of the fibres is in consequence more 

 perfect. A black cross is seen between crossed nicols, though, 

 owing to the slightly oblique extinction of the felspar which is the 

 dominant mineral-element, the arms of the cross are not very perfect 

 nor accurately at right angles. The structure is thus the pseudo- 

 spherulitic rather than the truly spherulitic of Hosenbusch, but from 

 our point of view the distinction is not essential. 



Graphic intergrowths are not always restricted in this rock to 

 the quartz and felspars : exceptionally they affect the other con- 

 stituents also. Certain slides show a radiate grouping of the minerals 

 on rather a large scale (relatively to the field of the microscope), 

 imperfect prisms of felspar being arranged like the spokes of awheel. 

 In this case the green augite occurs in irregular rod-like and elon- 

 gated patches sharing the general radiate disposition [2280]. Again, 

 there are not wanting indications of a micrographic intergrowth on 

 a small scale between augite and the other constituents, including 

 magnetite, though the decomposition of the augite renders the 

 appearance rather obscure. So far as I have observed, this structure 

 is found only in the remarkable basic modification of the granophyre 

 near its junction with the gabbro. 



The following analysis of the Carrock Pell rock is due to my 

 friend Mr. George Barrow, F.G.S., of the Geological Survey of 

 Scotland. It may be taken as representing the composition of an 

 average specimen ; the specific gravity was found to be 2*670 : — 



III. 



SiO., 71-60 



Alo0 3 1360 



Fe 2 3 2-40 



MgO 021 



CaO 2-30 



Na 2 555 



K s O 353 



Loss on ignition 0"70 



99-89 

 III. Granophyre, 100 yards east of summit, Carrock Fell. The iron is estim- 

 ated as ferric oxide, but doubtless occurs mostly in the lower form : 

 the figure given corresponds to 2 - 16 of ferrous oxide. 



The analysis by Mr. J. Hughes given in Mr. Ward's paper 1 shows 

 a general agreement with the above, except as regards the soda, 

 which must be largely overestimated. 2 



1 Quart. Journ. G-eol. Soc, vol. xxxii. (1876) p. 24. 



- It is evident that, when the only alkali-bearing minerals are felspars, the 

 molecular ratio K 2 0-)-Na 2 : A1 2 3 in the bulk-analysis of a rock cannot be 

 greater than unity, and must be less if a lime-benring felspar or an aluminous 

 pyroxene be present. The ratio calculated from Hughes's figures is 1/7 *. 1. 



