TRANSACTIONS OF SECTION C. 721 



least have been carried forward involved in the matrix, and glaciated chiefly by its 

 particles. Under the microscope the particles exhibit most of the varieties of form 

 and glaciation that are found among larger boulders. The structure of the till 

 in many open situations shows that the axes of its stones have been turned by a 

 common force in the direction of glaciation ; exhibiting a rough arrangement com- 

 parable to the fluxion-structure of igneous rocks, the smaller boulders dividing 

 around and apparently drifting past the larger, like the tide round an anchored 

 skiff. This structure, which has been found by the author over many hundreds 

 of square miles, chiefly in the North of England, indicates that at least a surface- 

 layer of tbe till was dragged along, with a shearing movement of particle upon 

 particle, producing intimate glaciation within its mass. Proofs are adduced that 

 this moving layer was in general a surface-layer only, and that the till did not, as 

 has often been supposed, move forward en masse, licking up its additions from 

 beneath. This appears to be the only intelligible explanation of the order (as 

 well as the structure) of the boulder-clays of which the author has any practical 

 knowledge. In up-lying situations, where the drift consists of raw material, 

 fluxion-structures are seldom to be detected. In sheltered spots they are not gene- 

 rally developed. They are characteristic of well-kneaded till in open situations, 

 liable, however, to obscuration by contortions within the mass. Of twelve experi- 

 mental attempts made near the watershed of England in East Cumberland, 000-000 

 feet above the sea, to determine the ice-movement from this structure alone, eight 

 were correct, three indeterminate, and only one misleading. The pressure and 

 movement capable of producing this widespread fluxion-structure seem to have 

 been that of some mass vast and far-spreading— closely investing, slow-moving, 

 and heavily dragging — such as glacier-ice. It needs only to be assumed that the 

 confluent glaciers communicated something of their own movement and structure 

 to the ground-moraine below. 



5. On the Glacial Origin of Lake Basins. 

 By Alfred R. C. Selwyn, LL.D., F.E.S. 



As we are all aware, a great deal has been said and written on the manner in 

 which the numerous rock basins now holding lakes have been formed, and that 

 they have been assigned by Ramsay and many other geologists to the scooping 

 power of ice. 



In 1870, Ramsay, Hull, James Geikie, Judd, Fisher, Bonney, and Hugh 

 Miller took part in the discussion of this question. 1 



On December 20, 1870, following the very interesting remarks on this subject 

 by the authors above named, I wrote as follows : * 



' In reading the correspondence and remarks on the origin of Lake basins, in the 

 November and other numbers of the ' Geological Magazine,' it has occurred to me 

 that the glacial origin of these basins may be explained without supposing the ice 

 to have scooped them out of solid ?-ocks, such as we now see around them. I have 

 been led to this idea by a study of the phenomena connected with the decomposi- 

 tion of rocks in situ in southern latitudes — Australia and Brazil. 



' Similar facts, I am informed, may likewise be seen in South Carolina, 

 Georgia, &c. 



' In these southern regions, which have never been glaciated, the surface over 

 more or less extensive areas consists of quite soft decomposed rock, and mining 

 operations have shown that the decomposition has been, from some unexplained 

 cause, very irregular in its action, and that often great masses resembling boulders 

 are quite unchanged though completely surrounded by the decomposed material ; 

 and the varying depth to which the decomposition has extended has resulted in 

 producing an underlying solid rock surface, as full of hollows and depressions of all 

 shapes and sizes as can be found in any of our northern lake regions. And if we 

 admit that prior to the Glacial period these northern lake regions were similarly 



1 Geological Magazine, vols. iii. and iv., 1876 anl 1877. 

 - Geological Magazine, vol. iv., p. 93. 



188 1. 3 A 



