part 3] lower paleozoic op artrog-dolgelley. 295 



I. Granophyre, ' eurite ' ; Llyn-y-Gader, Cader Idris. G. A. J. Cole & A. V. 

 Jennings, Q. J. G. S. vol. xlv (1889) p. 435. 

 II. Granophyre, ' granite ' ; Tan-y-grisiau, Ffestiniog district. A.V.Jennings 

 & G. J. Williams, Q. J.G.S. vol. xlvii (1891) p. 381. 



III. Granophyre; Trevor, Yr Eifl (Carnarvonshire). 'Summary of Progress for 



1918 ' Mem. Geol. Surv. 1919, p. 8. 



IV. Granophyre; 100 yards east of summit, Carrock Fell, Lake District. A. Har- 



ker, Q. J.G.S. vol. li (1895) p. 129. 

 V. Granophyre; Brandy Gill, Carrock Fell. A. Holmes, Geol. Mag. 1917, p. 406. 



The resemblance between these rocks is very close. No. V is 

 exceptional as regards its silica-percentage, but in this case the 

 sample was especially chosen as being a siliceous rock. 1 Soda is in 

 excess over potash in all cases, with the exception of the Tan-y- 

 grisiau rock, and calculations show from 50 to 60 per cent, of 

 plagioclase which varies between Ab 6 - 5 An I and Ab 7 - 5 An I. 



(/3) Marginal modifications of the Crogenen grano- 

 phyre. — The most basic of the marginal rocks are dark, and 

 resemble at first sight a fine-grained diabase. Like numerous 

 basic rocks they may show distinct traces of banding, and, apart 

 from the banding, there may be a patchiness due to an irregular 

 intermixture of acid and basic material. They weather in most 

 respects like the diabases, but the presence of scattered micro- 

 porphyritic felspars exactly resembling those in the normal grano- 

 phyre betrays their true nature (see PL XIX, fig. 3), as does 

 also their frequent patchy character (PI. XIX, fig. 4), which is 

 not seen in the true diabases of the district. 



Under the microscope, the most basic types are seen to possess 

 a rather granular texture, distinctly coarser than that of the 

 normal granophyre (PL XIX, figs. 2 & 3). They consist [type 2, 3 

 C 211] essentially of hypidiomorphic felspars and very abundant 

 dark minerals, which include principally a ragged pale-green horn- 

 blende and granular magnetite. Granular sphene and aeicular 

 apatite are both extremely plentiful as accessories. In such cases 

 quartz may be completely wanting. 



The felspar in these rocks, as in the normal granophyres, includes 

 both plagioclase and orthoclase. The latter crystals are quite 

 allotriomorphic, and usually destitute of any kind of twinning. 

 Carlsbad twinning may, however, be seen in some cases ; also the 

 characteristic streaky polysynthetic twinning of anorthoclase occa- 

 sionally makes its appearance, affecting either the whole of a 

 crystal or a part only. Again, therefore, the non-plagioclastic 

 felspar must contain a considerable amount of soda. The plagio- 

 clase felspars tend to be more idiomorphic than the orthoclase, but 

 not to so marked an extent as in the granophyre proper. Extinc- 

 tion angles and refractive indices show the felspar to be albite T 

 hence the differentiation of the magma has not affected the com- 

 position of the felspars. Both varieties of felspar are in fresh 



1 A. Holmes, Geol. Mag. 1917, p. 403. 



2 The types 1, 2, & 3 are taken in order from the exterior inwards. 



