94 PALAEOZOIC TIME. [CHAP. VI. 



Dean, while in Grower and Devon they are represented by 

 shales and fine-grained sandstones. In Ireland the Mill- 

 stone Grit is seldom coarse-grained, but usually consists of 

 flagstones which are never more than 650 feet thick. Such 

 being the arrangement of these grits, it is difficult to see 

 from what quarter they could have been derived, except 

 from the central and eastern parts of the large island 

 shown on PI. IV., these parts being probably its highest 

 and steepest districts. 



Even then, however, we cannot understand the super- 

 position of coarse sandstones upon a limestone and shale 

 series without assuming that great changes took place in 

 the physical geography of the area. During the formation 

 of the lower limestones the brooks which drained this part 

 of the land can hardly have carried any sediment to the 

 sea, their waters must have been clear, and the rainfall 

 must have been small; later on, however, they seem to 

 have carried both mud and sand, and eventually their 

 volume and velocity were so increased that they could 

 transport coarse sand to very great distances. I see only 

 one way in which such a change could be brought about, 

 and that is by a general and considerable elevation of the 

 area, raising the central parts of the island into those 

 atmospheric regions where rain, frost, and wind are most 

 vigorous and incessant in their action. The effects of up- 

 heaval in altering the character of the sediment deposited 

 round the land would be greater in the case of a rocky 

 island with steep slopes, which were prolonged beneath the 

 surrounding sea, than on the mainland, where the mountain 

 ranges would seldom be so close to the coast-line. In the 

 latter case much of the sand would be deposited before the 

 rivers reached the sea ; but in the former the streams and 

 torrents would not only be able to carry the sand to the 

 sea, but the issuing currents would sweep it to some dis- 

 tance from the shore. Further, not only would the velocity 



