112 Floristik, Geographie, Systematik etc. 



are reached. Some rivers traverse deep Channels and enter* the sea 

 abruptly through narrow gorges in the cliffs, others like the Wick 

 and Thurso rivers flow over alluvial lands near sea-level. The 

 rocks are mainly cristalline schists and granites, or sandstones and 

 calcareous flagstones of the Old Red Sandstone. These are extensi- 

 vely covered with glacial deposits, in the north-eastern half of the 

 area a tenacious shelly boulder clay rieh in lime predominates, 

 while in the inland south-western part there is a less tenacious 

 sandy boulder clay. These two types of glacial deposit have a deter- 

 mining influence on the Vegetation, and their distribution is shewn 

 on a small map, which also shows the area under eultivation, and 

 otherwise serves as a key-map. Peat Covers more than two-thirds 

 of Caithness, and formely was more extensive. The conditions 

 determining the formation of peat since early post-glacial times, and 

 the changes in the Vegetation of the peat leading on to the present 

 period of retrogression are discussed at considerable length. The 

 chief factors which have favoured the aecumulation of peat are: 

 1) the plateau-like topography and its influence on the prevalence 

 of winds, rainfall and drainage; 2) the condition of the surface of 

 the land at the retreat of the ice-sheet; 3) the latitude of Caithness 

 and its geographical position relative to the edse of the Continental 

 shelf. The subsequent history of the Vegetation is indicated by plant- 

 remains in the peat: the first tundra-like plant associations followed 

 by bog or dwarf scrub of Betula, then by a forest period with Pinus 

 which subsequently disappeared. Such changes are regarded as 

 possible only under the influence of climatic changes. In recent 

 times the oecurrence of extensive Stretches of peaty moorland has 

 acted as a barrier on the landward side to all plants incapable of 

 competing with moorland associations, so that plant-migration has 

 taken place mainly along the coast and river- Systems, and by the 

 aid of man. 



A review of the plant communities (Chap. II) is noteworthy 

 because in it there is outlined a grouping of plant formations of 

 considerable interest to the phytogeographer. The author suggests 

 "that dominant plant formations, such as the moorland (in Caith- 

 ness), which oecupy ground comparatively stable from the geolo- 

 gical Standpoint, should be termed 'stable' or 'palaeogeic' formations, 

 since the ground they cover mainly owes its features to past geolo- 

 gical processes; while for the limiting and dissecting plant forma- 

 tions, often found in all stages of progressive association and suc- 

 cession, from the migratory nature of the geological agents of 

 erosion and deposition, he suggests the terms 'migratory' or 

 'neogeic' formations, since the ground they oecupy owes its features 

 to recent geological processes." In Caithness this mode of treat- 

 ment resolves the Vegetation into one dominant stable formation, 

 the moorland, and several migratory formations in the belts along 

 the sea margin, the ramifications of the drainage System, and round 

 alpine centres. These formations are dealt with in detail with 

 numerous examples of associations in representative localities, the 

 floristic lists including many of the commoner Bryophyta. Within 

 the limits of an abstract only the briefest notes must suffice to 

 indicate the main plant communities. 



I. Alpine and Subalpine Plant Formations (Chap. III). 



1. Plant formation of the frost debris of the Alpine plateaux. 

 Habitat post-glacial in origin, subjeet to wind erosion and frost; the 

 debris has great porosity and low water capacity. Plant associations 



