448 



MR. A. WADE ON THE LLANDOVERY AND [Aug. I9II, 



finding the felspathic constituent to be albite, suggests that the 

 rocks are not diabases, as Prof. Watts had called them, but kerato- 

 phyres. In order to settle this point with regard to the rock of the 

 Welshpool Dyke, I first made several tests with the felspars. Using 

 Becke's method, I obtained results which indicated albite. Not 

 being satisfied, I employed microchemical tests. By means of 

 Boricky's methed — using a 4 per-cent. solution of hydrofluosilicic 

 acid — I obtained weakly double-refracting hexagonal crystals of 

 sodium silicofluoride (Na 2 SiFl 6 ) in great numbers, together with 

 isotropic cubes and octahedra indicating potassium silicofluoride. 

 Finally, I decided to make a series of analyses of the whole rock. 

 The results indicate a rock nearer to bostonite than to keratophyre, 

 and this would agree better with the microscopic characters. The 

 absence of porphyritic constituents, the presence of marked flow- 

 structure, and the fact that the rock is intrusive, are all in favour of 

 it being called a bostonite, and against the term keratophyre 

 being used in connexion with it. Mr. Jevons, in fact, states that 

 an occurrence described by him from the Berwyn Hills is probably 

 the first recorded of an intrusive keratophyre, while bostonites 

 typically occur in dykes. I append the analyses, together with 

 those of recognized bostonites obtained from two other localities : 

 it will be seen how closely the results agree. 



Comparison between Analyses of the Welshpool-Dyke Rock akb 

 similar Rocks from other Localities. 



Constituents. 



Welshpool 

 (No. 1). 



Welshpool 

 (No. 2). 



Bostonite. 

 Cowal} 



Bostonite. 

 Orkneys} 



Si0 2 



54-17 

 12-96 

 509 

 6-49 

 2-94 

 900 

 1-52 

 5-64 

 0-62 

 1-09 



54-15 

 17-97 

 2-06 

 5-50 

 066 

 1044 

 1-42 

 5-55 

 064 

 1-25 



56-4 

 190 

 3-5 

 4-8 

 1-5 

 2-6 

 4-5 

 5-0 

 2-6 



52-00 

 18-06 

 2-18 

 5-14 

 2-84 

 4-59 

 3-78 

 4-68 

 1-84 

 0-98 

 3-59 

 0-25 



A1 2 3 



Fe 3 



FeO 



MgO 



CaO 



Na ; 



K 



H 2 



Ti0 2 ., 



C0 2 



MnO 





Totals 



99-52 



99-64 



99-9 



99-93 





1 E. H. Hatch, 'Text-Book of Petrology' 5th ed. (1909) p. 231. 



