390 PAST succession: the ceneosere. 



together, and contends that species of the same formation can invade at 

 different times. 



Schroter (1904 : 344) has given a concise but comprehensive discussion of the 

 postglacial horizons of Switzerland, and the significance of peat-bogs in their 

 correlation. The account is so full, and the citation of the work of many 

 investigators so copious, that an adequate abstract is impossible, and hence 

 only the outline can be given : 



Basic geologic and climatological principles. 

 Plant and animal fossils, and prehistoric remains. 



Flora of the last interglacial and interstadial periods. 

 Glacial flora of the Dryas period. 

 Prehistoric remains. 



Gradmann's theory of the colonization of steppe-like regions. 

 Simunary of plant fossils. 



General conclusions upon the employment of the subfossil peat flora for reconstruction. 

 Age of the moors. 

 Age of the layers. 

 The moor as an historical record.- 

 Correlation of the living flora with the course of development. 

 Summary of the succession. 

 Detailed account of the succession. 



Lewis (1905:721) has summarized the general sequence of vegetation 

 observed in the peat of the Southern Uplands of Scotland, as follows: 



"The Merrick and Kells mosses, and the mosses in the Tweedsmuir district, 

 occur above and upon the moraines of the local glaciers of the Southern 

 Uplands, and must, therefore, be of later date than these. 



"That these mosses began to grow at some period between the disappear- 

 ance of the local glaciers and the reappearance of glacial conditions, is shown 

 by the presence in both districts of an Arctic plant-bed running between the 

 lower and upper woodland bed. The conditions which would favor the growth 

 of such a vegetation in the southwest of Scotland at only 800 to 1,200 feet 

 would be severe enough to cause considerable glaciation in the Highlands. 

 The plant-beds below and above the Arctic bed also tend to show that this 

 layer indicates one of the smaller and later returns to glacial conditions; for 

 the beds below show a gradual increase, and above, a gradual decrease, in 

 precipitation. If this reading is correct, interest would attach to an exami- 

 nation of any deep peat deposits resting on the 50-feet raised beach, as we 

 might expect to find, in that case, the representative of the Arctic zone of the 

 Merrick mosses resting upon the surface of the beach. 



"The peat of the Moorfoots contains no widespread forest beds, basal 

 birch only being foimd low down on some of the hill-sides. Eriophorum and 

 Molinia have been found at the base of the peat on the steepest hill-sides, thus 

 showing that these mosses began to form under extremely wet conditions, the 

 higher ground being covered with Eriophorum bog, wlulst the lower slopes 

 supported copses of birch and willows. There is no sign of Arctic vegetation 

 at the base of this peat, but the basal swamp vegetation gives place above to a 

 formation indicating much drier and probably colder conditions, represented 

 by a zone of Empetrum with Arctostaphylus uvanursi. 



"The question arises whether this Empetrum bed can be correlated with 

 the Arctic zone of the Merrick and Kells mosses and the Tweedsmuir peat. 

 If it is contemporaneous, then the later return to cold conditions represented 

 by the high level corrie moraines of the Highlands produced little effect upon 

 the vegetation so far south as the Moorfoots, for there are no beds above the 

 Empetrum zone in this peat which show any return to cold conditions. 



"The lowland mosses of Wigtonshire occupy large hollows in the till between 

 the outcrops of Silurian rocks, and reach a depth of about 20 feet. No Arctic 



