B. H. Rastall— Boulders in Cambridge Driff. 543 



kindness of Professor T. McKenny Hughes, F.R.S., about fifty of the 

 more promising types have been sliced. 



An examination of this collection shows that a large number 

 can be definitely identified as belonging to certain petrographical 

 districts. 



Many specimens are clearly referable to the Devonian soda-bearing 

 intrusions of southern Norway ; in this connection special mention 

 must be made of the rhomb-porphyry, of which a considerable 

 number of typical examples have been collected from the district 

 lying to the east, south, and west of Cambridge. 



A rather coarse-grained pink rock from Pampisford consists of 

 quartz, felspar, and abundant ferromagnesian minerals. Practically 

 the whole of the felspar is pei'thite of various kinds. The most 

 common coloured mineral is a green pleochroic soda-pyroxene, 

 which is moulded in a characteristic way on the quartz. There 

 are also a good many small crystals of a deep blue, intensely 

 pleochroic mineral, with an extinction angle up to 14°, This 

 sometimes occurs in parallel intergrowtli with the pyroxene. It 

 is identified as arfvedsonite. This slice agrees, down to the 

 minutest details, with Brogger's soda-granite, from the Christiania 

 district, 



A specimen from Newnham shows quartz, felspar, and long 

 needles of ferromagnesian mineral. The felspars are very variable 

 in character, and include all types of perthite, and especially 

 microcline-perthite. The coloured mineral is chiefly eegirine ; it 

 occurs in very long needles, with a very low extinction angle. 

 There is also a small quantity of arfvedsonite, like that in the 

 rock last described. These two rocks are evidently very closely 

 related. 



Another coarse granitic rock from Newnham contains brown 

 biotite and arfvedsonite, and is evidently nearly related to Nord- 

 markite. A slice from Barnwell shows the peculiar amphibole 

 described by Brogger as kataphorite, along with perthite and 

 abundant nepheline. Many other examples contain similar charac- 

 teristic minerals, and give evidence of relationship. 



Besides the above-mentioned rhomb-porphyries, other acid and 

 intermediate intrusives ai'e very common, and in thin slices they 

 often show characters that seem to connect them with the Christiania 

 familj^ 



The commonest type of quartz-porphyry shows large corroded 

 hexagons of quartz and phenoci'ysts of perthite, together with some 

 ferromagnesian mineral now represented by irregular aggregates 

 of deep brown and strongly pleochroic biotite. This method of 

 alteration also occurs in some other slightly different rocks. The 

 groundmass is microcrystalline, and often micropoecilitic. This rook 

 is identified by Professor Sjogren as coming from Dalecarlia. 



Another common porphyritic rock contains abundant phenocrysts 

 of felspar, both perthite and plagioclase, with a few rounded quartz 

 crystals, in a microcrystalline groundmass of quartz and felspar, 

 with numerous radiating groups of minute crystals of tourmaline, 



DECADE V. — VOL. I. NO. XI. 32 



