336 



SURFACE GEOLOGY. 



dividing line deeper down. Tlie fact of the presence of more iron in the clay than in 

 the till is readily supposable, since iron is cjuite likely to be concentrated with the finest 

 materials in the lowlands, leaving coarse pebbles behind. This, I think, answers all 

 your questions, as I had understood the matter ; but you will be the best judge as to 

 the correctness of the suppositions. 



The second letter is as follows: 



In reference to the questions you propose in your last letter, you understand that I 

 am unfamiliar with the field facts and the geological relationships involved, and so am 

 not very able in my opinion. As I understand you, your opinion is, that the lower till 

 was made from the clay derived from the grinding of the rocks by the lower surface of 

 the glacier, and the upper till from the rubbish in and on the glacier, which was depos- 

 ited on the melting of the glacier ; and that subsequently the lower blue clay was de- 

 rived from the lower till, and the upper gray clay from the upper till, by transportation 

 by water; and you put the questions, " Could the lower blue clay have been derived 

 from the lower till by deposition in water?" and " Could such material be transported 

 without oxidizing the iron?" It does not seem to me that the lower clays could be 

 derived from lower till, for if the washing took place after the deposition of the till, the 

 strata of the till would be inverted in the clay beds, and the upper till would make the 

 lower clay, since it would be the first to be transported by erosion. I do not think that 

 material could be transported without any oxidation of ferrous compounds ; but as the 

 lower blue clay contains ferric oxide, this is not necessarily supposed. I incline to my 

 former opinion of the production of an upper gray layer, in both cases, by those pro- 

 cesses of oxidation,— by the external agencies that are everywhere going on in the 

 rocks as well as soils. If, now, as is believed, the end of the glacial period was accom- 

 panied by great floods, and the latter end of the melting was rapid, the material in and 

 through the glacier would not be deposited on the ground on top of the lower till, but 

 would be washed away, and concentrated in the gravel beds. Clays in some cases may 

 have resulted from the deposition of the finer portions, assorted by the rapidity of cur- 

 rents ; but the material is usually coarse that rests on and in glaciers, and composed of 

 broken fragments. The fine clay is that which is ground and pulverized by the lower 

 surface grinding the underlying rocks. If, then, the till on the disappearance of the 

 glacier was a substance uniform in composition from top to bottom, and of the same 

 nature as the present lower till, then, by the action of running water, material from 

 this bed might be transported to form clay beds without further oxidation of the iron 

 than is shown by the difference between the lower till and lower blue clay. If the beds 

 of both till and clay, as first deposited, were of the same nature as the lower beds at 

 present, atmospheric and aqueous agencies would in time produce the differences in 

 both now seen. 



In speculating upon this subject, I was at first troubled by the some- 

 what similar origin of the upper till and blue clay, both in connection 



