280 REVOLUTIONS IN niYSICAL GEOGRAPHY. cuw. xiv. 



tho iiiioccno jicriod, like Madeira, Porto Santo, and tho 

 Dcsertas, constituting the small ^ladeiran Archipelago, we 

 might have expected to discover a diflferenco in the species 

 of land-shells, not only when Ireland was compared to Eng- 

 land, but when different islands of the Hebrides were con- 

 trasted one with anollier, and each of them with England. 

 It would not, however, be necessary, in order to eifect the 

 complete fusion of the animals and plants which we witness, 

 to assume that all parts of the area formed continuous land 

 at one and the stinic moment of time, but merely that the 

 several portions wei-e so joined within the post-pliocene era 

 as to allow the animals and jilants to migrate freely in suc- 

 cession from one district to another. 



Southernmost Extent of Erraties in England. 



In reference to that jiortion of the south of England which 

 is marked hy diagonal lines in the map at ]>. 270, the theory 

 of its having been an area of dry land during the period of 

 great submergence and floating ice does not depend merely 

 on negative evidence, such as the absence of the northern 

 drill or boulder-clay on its surface ; but we have also, in favor 

 of the same conclusion, the remarkable fact of the presence 

 of erratic blocks on the southern coast of Sussex, implying 

 the existence there of an ancient coast-line at a period Avhen 

 the cold must have been at its height 



These blocks are to be seen in greatest number at Pagham 

 and Selsea, fifteen miles south of Chichester, in hit. 50° 

 40' K 



Thej- consist of fragments of granite, syenite, and green- 

 stone, as well as of Devonian and Silurian rocks, some of 

 them of large size. I measured one of granite at Pagham, 

 twenty-seven feet in circumference. They are not of north- 

 ern origin, but must have come from the coast of Nor- 



