From William Hopkins   Dec. 18/61

Cambridge | Dec. 18/61

Dear Tyndall,

In writing my memoir on Glacial motion,¹ I want more precise information on one point than I can gain from existing books. I want the means of judging how far the veined structure in the lower part of a glacier is a structure transmitted from a higher part (where it was first generated) but modified as to the forms of the lines of structure by the subsequent motion of the glacier—or, whether it is more especially due to the direct and immediate action of the pressure to which the ice becomes subjected in the localities which any assigned portion of it occupies during its motion. The glacier proceeding from the Géant² with its dirt bands must afford I think one of the best tests to decide this question. I have no doubt as to your theory of the dirt bands.³ According to it they must be formed in positions very nearly coincident with the original veins or lines of structure at the foot of the ice-fall. Now since your dirt curves must necessarily be transmitted curves, deformed into long loops by the motion of the glacier, the same must probably hold with respect to the lines of structure if they remain in coincidence with the dirt bands. Is such the case or not? Forbes’s theory of them⁴ would make them necessarily so, but would not make them, I imagine, (I never can thoroughly understand any of his theories) so entirely transmitted forms as yours. And there it is manifest that his observations are loose and inaccurate.

The question of the transmission or instantaneous formation of the veined structure has an important bearing on any theory of the longitudinal structure under the central moraine arising from the confluence of two glaciers. If that structure be generated by the more immediate action of the physical causes to which it is referable at the junction, then Forbes’s theory cannot possibly be true: but may have a little toe, as it were, to rest upon if the structure may be considered as transmitted from the confluent glacier. The glacier of the Aar seems the most important case. Can you tell me how far below the Abschwung⁵ the longitudinal structure is fully developed? Also how far in the case of the junction of the Léchaud and the Géant glaciers? Certain cases of sudden beginnings and sudden terminations, of portions of the glacier where the structure is forcibly developed, afford an argument against the fact of transmission, at least to any considerable distance.

Yours truly | W. Hopkins.

RI MS JT/1/TYP/2/642–643

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