246 The Scottish Naturalist. 



would be accompanied by fragments of the hard igneous rocks 

 which overlie the Cretaceous strata of North Ireland. Chalk 

 and chalk-flints occur in the boulder clay of the Isle of Man, 

 where they are associated, Mr Home tells us, with CrirTel 

 granite and fragments of a dark trap - rock. 1 Possibly these 

 last are basalt-rocks from Antrim. It seems reasonable, there- 

 fore, to believe that erratics of Irish origin have found their way 

 to the Isle of Man ; and if this be so, it may be permissible to 

 assume that the chalk-flints of Blackpool, &c. (and perhaps also 

 some of the basalt-rocks), have come from the same quarter. 

 Mr Home has no doubt that the Irish erratics were brought 

 to the Isle of Man by land-ice. Referring to the conclusion 

 arrived at by Mr Close that the Irish mer de glace " was pro- 

 bably not less than 3000 feet in depth," he remarks, " It is 

 highly probable that this great mass of Irish ice succeeded, after 

 a hard battle {i.e., with the Scottish ice-sheet), in reaching the 

 Manx coast-line. It is not to be supposed that the normal 

 momentum of the respective ice- sheets remained constant. 

 The moving force must have varied with changing conditions. 

 On the other hand, it is quite possible that there may have been 

 an 'undertow' of the ice from the north east coast of Ireland, 

 which would easily account for Antrim chalk and chalk-flints in 

 the Manx till." I would go further, and state my conviction 

 that before the united ice-sheets had attained their maximum 

 development, it is almost certain that the ice flowing into the 

 Irish-Sea basin by the North Channel would for a long time 

 exceed in mass the coalescent glaciers that descended from the 

 southern uplands of Scotland, and would therefore be enabled 

 to extend much further to the east than it could at a later date, 

 when the general mer de glace had reached its climax. It might 

 thus have advanced as far as and even beyond the Isle of 

 Man. This inference is based upon the simple fact that the 

 area drained by the mer de glace of the North Channel was very 

 much greater than the area extending from the watershed of the 

 southern uplands of Scotland to the Isle of Man. Erratics 

 from the north of Ireland would thus travel down the bed of 

 the North Channel, and eventually be distributed over a wide 

 area up to, and possibly even some distance beyond, the Isle of 

 Man. But as the Scottish and Cumbrian ice-flows gradually 

 increased in importance, the mer de glace coming from the 

 North Channel would be forced further and further to the 



1 'Trans. Edin. Geol. Soc.,' vol. ii. : 1874. 



