366 



D. MACKINTOSH ON" HIGH-LEVEL MARINE DRIFTS. 



VIII. — Concluding Remarks as to whether the Submergence 

 was caused by the Subsidence oe the Land or Rising of 

 the Sea. 



In favour of the extension to the district under notice of Dr. 

 Croll's theory of a rise of the sea-level by the melting of circum- 

 polar ice, and consequent displacement of the earth's centre of 

 gravity, and subsequent fall of the sea to its present level, it might 

 be argued that this theory would most readily explain the coincidence 

 in level between the marine drifts of Macclesfield Forest, Erondeg, 

 Moel Tryfan, and the Three-Rock Mountain, especially when we 

 take into account the great distance between the first- and last- 

 named localities, and the comparatively very small vertical distance 

 between the highest known shelly drifts and the present sea-level 

 (about 700 to 1). It would likewise account for variations in the 

 rate of submergence, by attributing the variations to changes in the 

 rate of circumpolar liquidation, resulting from changes in tem- 

 perature. Against the application of this theory to the district 

 under consideration, we must take into account the differences in 

 level between different parts of the land as it stood when the above 

 marine drifts were deposited, and as it now stands. These dif- 

 ferences would seem to indicate horizontal inequalities in the rate at 

 which the land emerged, as might be expected on considering the 

 extent of the submerged area. Thus the level of the greatest 

 height reached by the chalk in Ireland (as Professor Hull tells me) 

 nowhere exceeds 900 feet or 1000 feet. But at the time when 

 chalk flints were transported to Moel Tryfan the chalk in Ireland 

 must have reached to about 1350 feet above the present sea-level. 

 The greatest height to which Eskdale granite extends is 1286 feet ; 

 but at the time when when this granite was transported to Frondeg, 

 it must, in situ, have reached to at least 1400 feet. I have ventured 

 to make these few brief remarks in conclusion with the view of 

 stimulating to further inquiry on the subject, and partly because I 

 have reason to believe that so eminent an authority as Dr. Darwin 

 is now inclined to the opinion that many of the apparent changes 

 of level in the land were caused by changes of level in the sea. 



SUPPLEMENTARY NOTES. 



During the present month (April, 1881), having gone over the 

 ground described in this paper a fourth time, I am able to furnish 

 the following additional details: — On going south along the west 

 side of the axial north-and-south ridge of Minera mountain, I saw 

 numerous large and small angular stones (Carboniferous grit, sand- 

 stoue, and quartzose conglomerate) ; but I could find few or no 

 rounded stones, excepting among the felstone erratics, which must 

 have come from the west. After crossing the ridge and going in a 

 S.E. direction down the eastern slope of the mountain, I arrived at a 

 point nearly half a mile north of Erwau. Here it may be necessary 



