A SPHERULITIG DOLERITE FROM VRYHEID, NATAL. 381 



A Spherulitic Dolerite from Vryheid, Natal. 



By 



^V. C'aiaBg»lM'll «>«iiiitli, B.A., F.G.S.,' 

 Assistant in the Mineral Department, British Museum. 



With Plates XXVIII— XXX and Text-figure. 



The rock wliicli forms the suhjectof this note is remarkable 

 on account of a beautiful radial structure which is shown on 

 weathered surfaces. 



Specimens were collected by Mr. A. Cowley and forwarded 

 by him to tlje Natal Museum at Pietermaritzburg. Two large 

 specimens together with several fragments were sent by 

 Dr. E. Warren to the British Museum, and it is on these that 

 the following description is based. The following particulars 

 of the mode of occurrence of the rock were communicated to 

 Dr. Warren by Mr. Cowley. 



The rock occurs on a private farm, A^aalbank, in Hlobane 

 A'alley, twenty miles east of Yryheid, Natal. Here it is 

 exposed as two parallel sills intruded in coal-bearing beds 

 belonging to the Karoo system. Numerous large blocks 

 have become detached from the main mass, and it is on these 

 that the beautiful weathering effects are so well seen. 



As seen on fresh unweathered surfaces the rock is almost 

 black in colour, and is very compact and fine-grained. The 

 radial structure is only revealed by a peculiar silky lustre 

 which it gives to the broken faces. With the aid of a lens a 

 few minute prismatic crystals can be made out scattered 

 through the fine-grained ground- mass. 



The study of thin sections under the microscope shows that 

 ^ Comniiniicated l:)y permission of the Trustees of the British Miiseum. 



VOL. 2, PART 3. 26 



