40 The Geology of Cambridgeshire 



county, but is most abundant in the east. Masses of phos- 

 phate-bearing sandstone like the Spilsby Sandstone and 

 carrying similar fossils are common in the railway cutting 

 east of Bartlow, but have not been observed further to the 

 west. Rounded quartz and quartzite pebbles, red sandstone 

 and fibrous gypsum, probably Triassic, are common about 

 Comberton and Old North Road, and have been found to the 

 east. Friable and porous magnesian limestone has been 

 collected. Boulders of Carboniferous rocks, especially Carboni- 

 ferous Limestone and Ganister, are everywhere abundant, and 

 these are often large. The sandstones are usually rounded 

 or subangular, but the limestones show most beautiful ' glacia- 

 tion.' Undoubted Lower Palaeozoic rocks are notable by 

 their absence, but metamorphic rocks and banded gneisses 

 are quite common. Quartz-mica schists without felspar, 

 garnet-mica schists, basic hornblende schists and tourmaline- 

 quartz rocks are perhaps worth noting, but many types occur. 



Of igneous rocks basalt is the commonest, and many 

 varieties both with and without olivine have been found, 

 some being exceedingly like parts of the Whin Sill. 



Porphyrites probably come next after basalts in point 

 of abundance, and among them are some like certain Cheviot 

 rocks. Granites, diorites, rhyolites and a whole host of less 

 recognizable rocks, some being volcanic, are fairly common. 



Last, but not least in interest, we have a series of rocks which 

 is characterized by containing numerous porphyritic felspars 

 which in section almost always appear as rhombs. A few of 

 these are exactly like the famous rocks of the Ringerike 

 Plateau, west of Christiania, and are certainly Rhomb-por- 

 phyries, and it is probable that the whole series was once 

 Norwegian. The commonest of the series is a beautiful granite 

 porphyry with dihexagonal quartz crystals and pink rhombs of 

 felspars which weathers to a cream colour. The ferrornagne- 

 sian mineral is not really fresh enough to determine but seems 

 to have been augite. This rock is already recorded from such 

 scattered localities as Dullingham, Linton, Royston, Gainlin- 



