'I lie Geolo(f// of Pata<ionia. 543 



})orpliyry, rhyolito, obsidian, ophitic olivine-doleritcs, olivino-basalts, 

 and acid tiilis. 



The basalt-flows cover an enormous area. They slope gently 

 towards the Atlantic, and are cut off from the Cordillera by a 

 longitudinal depression. In the neighbourhood of Lago Colhuape 

 there seems to liave been a distinct centre of eruption, apart from 

 that which commences nearer the Cordillera. All that can be said 

 of their age is that they are older than the transverse depres- 

 sions of the Cordillera, and older than the glaciation of the eastern 

 slopes of that chain. 



The Tehuelche Pebble-Bed, which covers nearly the whole of 

 Patagonia, has been ascribed to marine action by some authors, by 

 others to glacial action. A third suggestion is the agency of big 

 rivers. Xo one of these agents alone could have produced the 

 observed phenomena : the origin was complex. The bulk of the 

 material was brought by glaciers from the Cordilleras to the sea, 

 which then covered the greater part of the pampas. As the sea 

 receded, it distributed the pebbles over the bottom, so forming a 

 continuous layer, such as now exists between the eastern coast and the 

 Falkland Islands. Torrents resulting from the melting of the glaciers 

 assisted in distributing the material from the Cordillera. Part of 

 the material on the present eastern coast was derived from islets of 

 quartz-porphyry in the Pleistocene sea. A great difficulty is that 

 no basalt-pebbles are found at Santa Cruz east of the flows. 



The drainage-system includes several eastward-flowing rivers and 

 numerous lakes, some of which occupy transverse valleys cutting 

 through the Cordillera. An example of the latter is Lago Buenos 

 Aires. The history of this lake can be gathered from the evidence 

 observed on its shores. Lagos Musters and Colhuape are two other 

 interesting lakes near the eastern coast. The width and depth of 

 the river- valleys are disproportionate to the present streams : this 

 can be explained by a decreasing rainfall, and also by the diversion 

 of many tributaries to the Pacific. Some valleys are dry, as, for 

 example, the Great Cafiadon Salado. 



3. ' On a Fossiliferous Band at the Top of the Lower Greensand, 

 near Leighton Buzzard (Bedfordshire).' By George "William 

 Lamplugh, Esq., F.G.S., and John Francis Walker, Esq., M.A., 

 F.L.S., F.G.S. 



This paper describes a newly-discovered fossiliferous band at the 

 top of the Lower Greensand, overlain by the Gault, in the sand-pits 

 at Shenley Hill near Leighton Buzzard, in Bedfordshire. The 

 fossils of this band present a different facies from that of any other 

 previously-known fossiliferous horizon of the Lower Greensand, and 

 show closer affinities with the fauna of the Upper Greensand than 

 have hitherto been recognized in any deposit below the Gault. The 

 brachiopoda are closely allied to those contained in the Tourtia Beds 

 of Belgium. The fossiliferous bed is rather sharply marked ofi' 

 from the underlying unfossiliferous ' silver-sands,' but is still more 

 sharply marked off from the overlying Gault. Stratigraphically it 

 forms part of the Lower Greensand, and cannot (without violence to 



