James Croll — Boulder-clay of Caithness. 211 



Theories Regarding the Origin of the Caithness-clay. 

 Mr. Jamieson, as wo have scon, freely admits tliat tlio Bouldcr- 

 olay of Caithness has tlio appoavanco of tnio laud-ico Till, but from 

 the N.W. and S.E. direction of the strias on the rocks, and the 

 presence of sea-shells in tho clay, he has come to the conclusion that 

 the glaciation of Caithness has been effected by floating ice at a 

 time when tho district was submerged. I have always felt con- 

 vinced that Mr. Jamieson had not hit upon the true explanation of 

 the phenomena. 



(1) It is physically impossible that any deposit formed by ice- 

 bergs could bo wholly unstratified. Suppose a mass of the materials 

 which would form Boulder-clay is dropped into the sea from, say an 

 iceberg, the heavier parts, such as stones, will reach the bottom first. 

 Then will follow lighter materials, such as sand, then clay, and last 

 of all the mud will settle down over the whole in fine layers. The 

 different masses dropped from the various icebergs, will, no doubt, 

 lie in confusion one over the other, but each separate mass will show 

 signs of stratification. A good deal of Boulder-clay evidently has 

 been formed in the sea, but if the clay be unstratified, it must have 

 been formed under glaciers moving along the sea bottom as on dry 

 ground. Whether nnstratified Boulder-clay may happen to be formed 

 under water or on dry land, it must in either case be the product of 

 land-ice (see Philosophical Magazine for November, 1868, p. 374). 

 Those who imagine that materials, differing in specific gravity 

 like those which compose Boulder-clay, dropped into water, can 

 settle down without assuming the stratified form, should make 

 the experiment, and they would soon satisfy themselves that the 

 thing is physical^ impossible. The notion that unstratified Boulder- 

 clay could be formed by deposits from floating ice, is not only 

 erroneous, but is also positively pernicious, for it tends to lead those 

 who entertain it astray in regard to the whole question of the origin 

 of drift. 



(2) It is also physically impossible that i'ce-markihgs, such as 

 those everywhere found on the rocky face of the district, and on the 

 imbedded pebbles and shells found in the clay, could have been 

 effected by any other agency than that of land-ice. I need not here 

 enter into any discussion on this point. This has been done at con- 

 siderable length on a former occasion — (see Philosophical Magazine 

 for November, 1868, pp. o66-374). A discussion on this point is, 

 however, unnecessary in the present case, because, if it can. be shown 

 that all the facts can be accounted' for in the most natural manner by 

 the theory of land-ice, no one will seek to contend for the floating-ice 

 theory ; for it is admitted that, with the exception of the direction 

 of the strige and the presence of the shells, all the other facts agree 

 better with the land-ice than with the floating-ice theory. 



My first impression on the subject was that the glaciation of 

 Caithness had been effected by the polar ice-cap, which, duiing the 

 severer part of the glacial epoch, must have extend.ed d'own to at 

 least the latitude of the north of Scotland. 



On a former occasion, see the " Header" for 14th October, 1865, 



