252 MESSRS. E. H. EASTALL AND J. ROMANES ON [Aug. I909, 



Prof. Hughes and other well-known authorities, and are beyond 

 suspicion. 



From this district it has been found possible to identify with 

 certainty a large number of rock-types, and many of them are of 

 unusual interest ; in particular, rocks from the Christiania petro- 

 graphical province are abundant. Tbe most distinctive of these is 

 rhomb-porphyry ; not more than five or six specimens of this have 

 been found actually in situ within the district here defined, but a 

 large number have been detected in buildings and pavements in 

 Cambridge, and some of quite large size, up to half a cubic foot in 

 volume : both the dyke and lava-facies of this rock are known. 

 Other members of this series include soda-granite and segirine- 

 granite from Kewnham, and from the Barnwell gravel a large 

 specimen of a very fine nepheline-syenite ; a slice of this was sub- 

 mitted to Dr. Flett, and was considered by him to show strong 

 affinity to the laurdalite group. Two specimens of nordmarkite 

 and one of nordmarkite-porphyry were also sliced : these have been 

 identified by Prof. Brogger. A large boulder of typical laurvikite 

 was collected many years ago by Prof. Hughes. 



In this district also we find in great abundance a rock which has 

 given a great deal of trouble, and is not yet identified. It is a 

 beautiful quartz-porphyry or granite-porphyry, with abundant 

 pink felspars and dihexagonal quartz-crystals set in a rather coarsely 

 crystalline ground-mass. It weathers in a peculiar and very 

 characteristic way. The weathered surface is -often remarkably 

 smooth, and commonly yellowish or pale brown in colour. The 

 hexagonal quartz-crystals show up with striking clearness as dark 

 spots. In the short paper in the Geological Magazine for 1904, 

 this was described on the authority of Prof. Sjogren as the Dalecarlia 

 porphyry (p. 542), but this identification appears to be erroneous. 

 Many attempts have been made to ascertain the origin of this 

 rock, though hitherto without success. This is peculiarly unfor- 

 tunate, since it is one of the most characteristic constituents of 

 the Cambridge Drift, occurring in large numbers in almost all the 

 districts investigated. In this paper it is referred to throughout as 

 ' porphyry x.' 



Another distinctive type is a dark felsitic rock containing a 

 few quartz-crystals and a good many pink felspars. Under the 

 microscope the rock shows phenocrysts of quartz, plagioclase and 

 alkali-felspar, some perthitic. The ground-mass is microcrystalline, 

 and contains abundant needles of tourmaline in radiating groups. 

 This type has been recognized by Messrs. Campbell and Gordon as 

 occurring plentifully in the conglomerates of the Old Eed Sand- 

 stone of Forfarshire. It will be referred to throughout as ' the 

 Forfarshire porphyry.' 



An interesting rock found in this district is a coarse-grained red 

 granite, represented by two blocks from Barnwell and Milton Koad 

 respectively. It consists of quartz, microcline-perthite, oligoclase 

 and biotite, with accessory zircon, and interstitial patches of an 



