280 REVOLUTIONS IN PHYSICAL GEOGRAPHY. CHAP. XIT. 



the miocene period, like Madeira, Porto Santo, and the 

 Desertas, constituting the small Madeiran Archipelago, we 

 might have expected to discover a difference 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 another, and each of them with England. 

 It would not, however, be necessary, in order to effect 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 same moment of time, but merely that the 

 several portions were so joined within the post-pliocene era 

 as to allow the animals and plants to migrate freely in 

 succession from one district to another. 



Southernmost Extent of Erratics in England. 



In reference to that portion of the south of England which 

 is marked by diagonal lines in the map at p. 260, 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 

 drift or boulder clay on its surface ; but we have also, in favour 

 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 when 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 

 lat. 5040'N. 



They 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 nor 

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



