498 J. O. Goodchild—On Drift. 



lower down alternations of till with sand and gravel begin to occur ; 

 and at a still greater distance from the head waters it often happens 

 that nothing but accumulation of water- worn materials make up the 

 whole mass of the drift. 



Where natural or artificial sections afford an opportunity of ex- 

 amining the internal structure of the drift mounds, we find that the 

 beds are often, perhaps nearly always, wrapped round a core of the 

 underlying solid rock ; and, what is still more remarkable, the bed- 

 ding planes of the outer parts of the deposit not rarely conform in 

 slope to the adjacent outer surface of the drift mound ; and this 

 holds good whether the outer part be clay and the mound form a 

 drumlin, or sands and gravels occupy the surface and the deposit 

 assume the form of an esker. 



In those places where the proportions of till and gravel are about 

 equal, most of the sections show lenticular deposits of laminated gutta- 

 percha clays, interbedded with the sands and gravels, and with both 

 the upper and the lower tills. The clays are often sufficiently fine 

 to exhibit a shining surface when cut, and are arranged in ex- 

 ceedingly thin leaves, forming masses which are occasionally from 

 ten to fifteen feet in thickness, but of very limited extent horizontally. 

 Notwithstanding their exceeding fineness, they are often strongly 

 false-bedded, in some cases up to angles of thirty degrees, or even 

 more ; and the false-bedded lamina frequently repose upon, and are 

 again covered up by, other laminee, which have other lesser degrees 

 of inclination, or are even quite horizontal. But the most note- 

 worthy feature about these sections is the intercalation of highly 

 contorted and crushed loams and laminated clays, forming bands 

 from a few inches to several feet in thickness, among perfectly 

 horizontal, or at all events clearly undisturbed strata. In some 

 sections these appearances are repeated four or five times in the 

 same mass, the beds between the crushed and contorted beds being 

 usually underanged. Where the clays in the till exhibit these 

 phenomena, the slickensided faces, which Mr. James Geikie has 

 termed " striated pavements," are usually to be met with. 



It is important that attention should be called to these laminated 

 clays, as they locally occur as well in both tills as in the middle 

 group ; and, clearly, no theory of the origin of the drifts can be 

 tenable if it do not also offer a satisfactory explanation of the enig- 

 matical phenomena referred to. 



Generally, the different members of the drift are so closely inter- 

 woven, so nearly resemble each other in regard to the direction of 

 transportal of the included stones, and the species of mollusca dispersed 

 throughout the series— where it is fossiliferous — so often exhibit the 

 same anomalous intermixture of Celtic, Lusitanian, Boreal, and 

 Arctic forms, that one cannot resist the conclusion that the whole 

 series must, in some way or other, have had a common origin. 



In framing any theory of the origin of drift we have therefore 

 to bear in mind the following points : — 



The component materials vary in size from mere grains of sand 

 up to blocks which are fifteen or twenty feet in diameter, or in 

 some cases even larger. 



