544 Rev. W. Lower Carter — On River Capture. 



Porphyritic lavas of intermediate and basic character are very 

 common in the district, and many of them can safely be referred 

 to the Old Eed Sandstone volcanic series of the Cheviots and central 

 Scotland. The most abundant is an enstatite-augite-andesite of the 

 usual character. 



A porphyrite fi-om Lord's Bridge is remarkable for having a gi*eat 

 variety of porphyritic minerals : the most prominent are a plagioclase 

 near to oligoclase, with a secondary boi'der of orthochxse, original 

 orthoclase, biotite, hornblende, and notably spheue ; the groundmass 

 is microgranitic, with a tendency to orthophyric structure. A few 

 rounded quartz grains also occur. 



Mr. G. Barrow has kindly looked over some selected slices, and 

 lie identifies half a dozen of these as belonging to the Garlton 

 plateau and Forth district. These include two specimens of anal- 

 cime diabase of the Forth Valley type. Some slices of olivine basalt 

 show the glomeroporphyritic structure which is so characteristic of 

 this series. 



A rock from Pampisford gi-avel-pit proves to be an unusually 

 fresh example of a very basic lava ; it is a porphyritic rock with 

 phenocrysts of olivine and augite in a groundmass of augite and 

 deep brown glass, with a few laths of felspar. This rock must be 

 classed as a limburgite ; its place of origin has not been identified. 



Enough work has not yet been done to enable any definite 

 conclusions to be drawn as to the glacial phenomena of Cambridge- 

 shire, but the facts here shortly summarised are sufficient to indicate 

 the presence in this area of a large number of the rock-types which 

 are so characteristic of the glacial deposits of other districts in the 

 east of England, and especially many of the best known Norwegian 

 rocks. So far no rock has been identified from the Lake District or 

 any region to the west of the central watershed of Great Britain. 



Vl. — EivEU Capture in thk Don System, 

 By the Eev. W. Loaver Carter, M.A., F.G.S.i 



THE river Don has a remarkable semicircular course. Eising in 

 the Middle Grits, west of Dunford Rriilge, at 1,500 feet above 

 CD., it flows eastwards to Penistone (700'), where it makes a bend 

 to the south-east, quickly deepens its valley to 500', and at Wortley 

 breaks through the great watershed (1000') of the Grenoside and 

 Wharncliflfe grits. It then receives the Little Don, the Ewden, 

 and the Loxley, on its right bank, and falls into the valley of 

 the Sheaf at Sheffield (150'). The Don then makes a rectangular 

 bend to the north-east, following the old valley of the Sheaf to 

 Conisborough, receiving the Eother on its right hank at Eotherham 

 (87') and the Dearne on its left bank at Denaby (45'). It then 

 traverses the Magnesian Limestone escarpment in a fine gorge, and 



^ Paper read before the British Association, Cambridge, Section C (Geoloir^"), 

 August, 1904. 



