446 APPENDIX H. 



tropical Africa, effectually prevents the northward extension 

 of most forms of animal and vegetable life ; while in a period 

 geologically recent, it is most probable tha' the same area was 

 occupied by a wide gulf, which served the same purpose of 

 barring the migration of southern forms. 



It may be premature to attempt to trace in further detail 

 the origin of the Great Atlas Flora ; but the facts already as- 

 certained certainly authorise some negative inferences. The 

 absence of plants of Arctic type proves that if some moun- 

 tains of Southern Europe received contributions to their 

 vegetation during the glacial period by means of floating ice- 

 rafts, that mode of diffusion did not extend to the Great Atlas. 

 If we suppose that during the glacial period the temperature of 

 the region north of the Atlas had fallen so low as to permit 

 the migration of northern species across the intervening low 

 country, we find it diflicult to understand why so many species 

 which, according to this theory, must have retreated to the 

 Atlas on the subsequent rise of temperature, should have failed 

 also to find a refuge in the mountains of Southern Spain. 



It is a further difficulty that if the constituents of the 

 Great Atlas Flora had, to a large extent, travelled by the route 

 here indicated, other species, now inhabiting the mountains of 

 Southern Spain, could scarcely fail to take the same road, and a 

 much nearer connection than is now apparent would have been 

 established between the Floras of these two mountain regions. 



It is, at least, possible that the wide diffusion of many of 

 the species constituting the so-called Germanic Flora may date 

 from a period much more remote than is ordinarily supposed ; 

 and it is a circumstance not without significance that so many 

 species of this type prove themselves capable of tolerating wide 

 variations in conditions of soil and climate. 



APPENDIX H. 



Notes on the Geology of the Plain of Marocco and the 



Great Atlas. 



By George Maw, P.G.S., F.L.S., &o. 



Of the Geology of Barbary little information has hitherto been 



put on record. The only publications with which I am ac- 



