McKahon and HutcMngs — On Pseudo-SjiJurulites. 257 



V. —Note on Pseudo-Spherulites. 



By Lieut. -General C. A. McMahon, V.P.G.S., 

 and "W. Maynard Hutchings, F.G.S. 



THE Jannai-y number of this Magazine contains an interesting 

 paper on a soda-bearing rock from Dinas Head, Cornwall, read 

 by Ml". Howard Fox, F.G.S., at the Oxford meeting of the British 

 Association. Mi\ J. J. Harris Teall, F.R.S., supplied notes on the 

 microscopic structure of some of the specimens ; and four chemical 

 analyses by Mr. J. Hort Player and Mr. Arthur F. Hosking are 

 given in the paper, from which it appears that the specimens 

 analysed are composed of from 64 to 66 per cent, of silica ; from 

 19 to 20 per cent, of alumina; 9-8 of soda; and small amounts 

 (generally under 1 per cent.) of potash, titanic acid, ferric and 

 ferrous oxides, magnesia and lime. 



A sketch of a nodular spherulitic specimen is given at page 18, 

 Geol. Mag. 1895. 



In a subsequent paper ^ read by Mr. Fox before the Royal 

 Cornwall Geological Society in 1894, other specimens from the 

 same locality, the microscopical examination of which was under- 

 taken by one of the authors of this paper, are described. 



Some of the soda rocks from Dinas Head, which form the subject 

 of this paper, contain a number of nodules closely resembling 

 spherulites to which it seems desirable to invite the attention of 

 petrologists. They are mostly in rounded forms, but owing to their 

 interference with each other's growth, thin sections of them, under 

 the microscope, sometimes present irregular hexagonal outlines. 

 Superficially examined they greatly resemble true spherulites, and 

 have been accepted as such by some good observers. 



The main question which arises in connection with these 

 spherulites is, whether they are of igneous origin, and denote 

 a vitreous igneous rock, or whether they represent contact action 

 exercised on a sedimentary rock. After a prolonged study of the 

 specimens sent to him, General McMahon came to the conclusion 

 that they are sedimentary rocks indurated and altered into a variety 

 of porcellanite, or adinole, by contact metamorphism. 



The structure of the base in which the pseudo-spherulites are 

 imbedded might be described as micro-granular or crypto-crystalline; 

 and it is not, taken alone, of sufficiently distinctive character to 

 enable a petrologist to answer the above question offhand. One 

 sees this structure in some altered sedimentary beds, and also in 

 some igneous rocks. In such cases one has, before arriving at 

 a verdict, to search for felspar prisms, or microlites, scattered about 

 in the base, and these are rarely absent in rocks of igneous origin. 

 In the rocks under discussion, if we exclude the pseudo-spherulites 

 from consideration, not a single lath-shaped prism of felspar, or 

 microlith, is to be found in the base. On the contrary, one of the 

 slides contains evidence which points decidedly to a sedimentary 



1 Notes on the Cherts and Associated Rocks of Eoundhole Point, Cataclews Point, 

 and Dinas Head. Trans. E.C.G.S. 1894. 



DECADE IV. VOL. II. NO. YI. 17 



