344 T. F. Jamieson — Changes of Level in the Glacial Period. 



is probably largely affected by faulting, they must be referred to 

 the Volcanic Series itself. 



In corroboration of our view that the Drygill Shales form part 

 of the Volcanic Series, we may draw attention for a moment to 

 the unequivocal occurrence of bands of mud stone — unfortunately 

 apparently not fossiliferous — in the lavas and ashes of this series in 

 another locality. The locality alluded to is Falcon Crag, near 

 Keswick, selected by Mr. Ward as affording a typical section of the 

 Volcanic Series (Geology of the English Lake-district, p. 13), though 

 he does not mention the particular feature here in question. Above 

 the lowest lava in this section, as seen in Cat Gill, we find a series 

 of unaltered ashes and breccias, sometimes fine, sometimes of coarser 

 grain, which alternate with thin bands of grey shaly mudstone. 

 These alternating beds are sometimes so mixed and blended together 

 at their lines of junction, that it is clear that the ashes were from 

 time to time abundantly showered into the sea in which the grey 

 mud of the shales was in course of deposition. The mudstones have 

 no resemblance lithologically to the Skiddaw Slates, and they are so 

 much squeezed and jointed that all attempts to detect fossils in them 

 have hitherto proved abortive. They are, however, full of ferruginous 

 stains and cavities which may possibly represent organic remains 

 •now destroyed. We have here, then, an unquestionable instance of 

 the occurrence of contemporaneous mudstones among the lavas and 

 ashes of the Volcanic Series ; and this would so far support our 

 reference of the Drygill Shales to the same series. At the same time, 

 it should be added that we are not disposed to regard the mudstones 

 of Falcon Crag as being the precise equivalent of the Drygill Shales. 

 On the contrary, the latter not only have a vastly greater thickness, 

 but are associated with a peculiar type of volcanic ejecta, of which 

 we find no representative in the section at Falcon Crag. 



III. — On Some Chakges of Level dtjeing the Glacial Period 



AND THEIR SUPPOSED CauSE. 

 By T. F. Jamieson, F.G.S., Ellon, Aberdeenshire. 



AT the close of the Glacial Period in North America the basin of 

 the Eed Eiver was occupied by a large lake which has received 

 the name of Lake Agassiz. It extended from what is now Lake 

 Winnipeg southward for some hundreds of miles to the watershed 

 of the Minnesota Eiver. The bottom of this old lake now forms an 

 extensive level plain famous for its fertility and the fine crops of 

 wheat it produces. 



At present the drainage of the Eed Eiver Valley flows northward 

 into Lake Winnipeg and from thence along the Nelson Eiver into 

 Hudson's Bay ; but during the existence of the lake in question this 

 outlet is supposed to have been blocked by the great glacier which 

 formerly covered all the region to the north, and which was then 

 slowly retreating owing to the advent of a milder climate. As the 

 drainage of the glacier went into the lake, there must have been 

 an ample supply of water to keep it constantly brimming. The 



