40 PROCEEDINGS 0¥ THE GEOLOGICAL SOCIETY. 



the western or occaiiic faces, the 100-fathom line of equal depth, 

 which is a different thing from a line or lines of greatest depth. To 

 which we can only answer, that the 100-fathom Hne is the greatest 

 depth for which the necessary soundings have heen given to enable 

 us to construct a continuous line for the whole distance. There are 

 indications of a line of greatest depth outside of this, and which may 

 be distinctly traced in the channel between the Perroe Islands and 

 the Orkneys, and between Eockall and the Hebrides ; but, as the 

 necessary soundings are not given for the southern part of its course, 

 we do not insist on it. 



This form of an irregular polygon, usually the pentagonal or the 

 hexagonal, is the form that bodies approximate to, more or less, in 

 shrinking, either when cooling down from a great heat or when 

 drying. Of the former process basaltic columns afford familiar ex- 

 amples ; and the same kind of thing may be seen in large surfaces 

 of river-mud drying under the influence of a hot sun ; and from 

 what we know otherwise, the probable inference is that the con- 

 traction or shrinking in question (from whence these large polygonal 

 areas appear to have had their origin) has arisen from cooling, and 

 the falling in of cavities occasioned by upheaval. 



The difference between the deep isolated pits and simple lines 

 of depression appears to be this, that in the one case the strata 

 are more unyielding than in the other. The area of the English 

 Channel has been shown to be a valley of depression, from the 

 terrestrial remains fished up in it, and the sunken forests on its 

 edges. The same thing is known of that part of the German Ocean 

 which is south of lat. 53°. There can then be no improbability in 

 assigning a similar origin to the northern part of it. We have, 

 therefore, two antagonistic forces in operation — the one an elevating 

 and expanding force, the other depressing and contracting, both 

 acting, if not in lines exactly straight, at least nearly so. If the 

 bed of the English Channel, east of the Start Point, were upheaved 

 30 fathoms or 180 feet perpendicular, it would present the ap- 

 pearance of a chain of lakes, similar to what is seen in the inland 

 valley (the Great Glen) through which runs the Caledonian Canal. 

 Does it not then appear probable that the latter vaUey has also had a 

 similar origin, lying as it does between two lofty mountain -chains? 

 The furrow has run parallel to the ridges on either side of it. That 

 it has been upheaved above the level of the sea by an after-process 

 may be inferred from this, that the narrow trough at the western 

 end of it (the Linnhe Loch), and the Moray Eirth at its eastern 

 entrance, have both the same maximum depth of water, viz. 100 to 

 120 fathoms. 



The lino of 100 fathoms on the western face of the islands is the 

 greatest depth at which numerous soundings are given, and thus 

 yields better data for a continuous line. 



We will commence at its north-eastern end, to the north-east of the 

 8h3tland Isles, where it forms a remarkable projection into the deep 

 water beyond. (1) From this point, it keeps a westerly direction 

 until north of the Shetlands, and then bends somewhat to the south 



