84 PALAEOZOIC TIME. [CHAP. VI. 



or cliff, against which the limestones and their associated 

 strata were laid down. 1 



Lastly, can we gather any evidence regarding the northern 

 limits of the Carboniferous sea and the probability of con- 

 tinental land existing in that direction ? There is, in the 

 first place, good reason to believe that land existed outside 

 the north-west portion of Ireland : in Galway and Mayo 

 the Carboniferous limestones are everywhere bordered by 

 conglomerates, and it is stated that these are on the horizon 

 of the Upper Limestones ; that near Oughterard the conglo- 

 merates and sandstones graduate eastward through shales 

 into limestones along the line of strike, and that in other 

 localities the limestones themselves contain pebbles up to 

 the size of a bean. 



The northern part of Donegal seems to have formed 

 part of this land, for the Carboniferous rocks occurring in 

 South Donegal and North Tyrone seem to pass northward 

 into shales and sandstones. 



On the west coast of Scotland, in the district of Morvern 

 (Argyleshire), there is a small tract of Carboniferous 

 sandstone let down by faults among the older rocks. This 

 is believed to be of Coal-measure age, and was probably 

 laid down in a gulf which penetrated the northern mass 

 of land between Donegal and the Highlands of Scotland ; 

 we may suppose, therefore, that a similar gulf existed in 

 Lower Carboniferous times, but a smaller and less exten- 

 sive one. 2 



There is every reason to suppose that the Highlands 

 were land throughout the Carboniferous period. I am in- 

 formed by Mr. H. M. Cadell that the oil-shales of the 

 Lothians are mostly replaced by grits and sandstones in 

 the north of Fife, and that everything indicates an approach 

 to land in that direction. Indeed, the persistent recurrence 



1 "Mem. Geol. Surr. Scotland, Expl. of Sheet 24," p. 17. 



a By mistake it has been carried too far north in the map, Plate IV, 



