Page:The Journal of geology (IA journalofgeology11893univers).pdf/96

between the successive layers of ice, as stated by Mr. Upham on pages 38-9 of the article referred to (quoted below). This differential motion arises from friction at the bottom and extends to the summit. It was natural, therefore, to take it for granted that the supposed rising current extended as far as its postulated cause. It was to be assumed, of course, that the current would rise less rapidly in the upper part if the difference of movement of successive ice layers were less there than below, but it would seem that the rise must be supposed to continue at some rate so long as the differential motion continued, i. e., until the surface was reached. The accession of snow-fall within the zone of accumulation would, to be sure, prevent erratics from reaching the new surface thus continually formed, but it would not prevent their reaching the surface in the zone of wastage. It is this latter zone with which our problems of deposition and many of our problems of derivation have chiefly to do. The career of some erratics is wholly confined to it. It goes without saying that ablation brings the surface down and is a factor in every exposure within the zone of wastage, but this does not prevent the erratics rising (by hypothesis) until they meet it. This conception of rising currents met by a plane of ablation I supposed without question to be that entertained by Mr. Upham and others. To be sure, in a strict and complete statement under this view the exposure of englacial erratics at the surface would be attributed to the joint result of the upward movement and the downward melting, but the liberties of brief and convenient statement would permit it to be referred to in terms of either factor, and I have interpreted the expressions of these writers on this basis. The correction does not, so far as I can see, in any serious way affect the main question under discussion. If there were rising currents bearing erratics to heights of 500 to 1000 feet above the base of the ice the result in ultimate deposition would be essentially the same as if the currents rose to the surface. If the rising currents are a misinterpretation, it is immaterial whether they be supposed to bear erratics to varying heights up to 500 or 1000 feet, whence these erratics move forward parallel with the base of the