56 



NORWEGIAN BOULDER IN THE MILLSTONE 

 GRIT OF YORKSHIRE. 



A. GILLIGAN, B.Sc.,, F.G.S. 

 Leeds. 



Among a remarkable suite of pebbles which the author has 

 obtained from the Millstone Grit series of Yorkshire occurs 

 one which has a striking resemblance in the hand specimen to 

 the well-known rhomb-porphyry of the Christiania district 

 which is so abundant in the Glacial deposits of E. Yorkshire. 

 The pebble when first obtained was roughly ellipsoidal in 



Photo by] [A . Gilligan . 



Showing obtuse angle of one of the large 



felspar crystals, and fine-grained groundmasB. 



x 20. 



form, the axes measuring 5x2^x2 inches. The broken surface 

 is lighter in colour than the common type of rhomb-porphyry. 



Under the microscope, it shows the following characters. 

 The groundmass is rather fine grained, and is made up of felspar 

 (microperthite), with rather abundant quartz. The accessory 

 minerals are sphene, zircon and apatite, but these occur very 

 sparingly. The phenocrysts are probably anorthoclase, but 

 are so decomposed that definite determination is impossible. 



The specimen and section have been submitted to Prof. 

 Brogger of Christiania, who writes as follows : — ' The rock 

 is not a rhomb-porphyry. It is, however, possible that it 

 belongs to the alkaline series of eruptive rocks of the Christiania 



Naturalist, 



