126 



VEGETATION OF THE PEAK DISTRICT 



[CH. 



PSEUDO-CALCAREOUS HEATHS 



The true calcareous heath should not be confused with the 

 alternation of lime-loving and humus-loving species where this 

 alternation is due to the occurrence of non-calcareous detritus 

 or drift in a chalky or limestone district. Such a tract of 

 vegetation is made up of the mixture of small societies or 

 associations belonging to two or more plant formations, just as 

 the terrestrial vegetation of small islands dotted about a lake 

 differs from the aquatic vegetation in the lake itself. The true 

 calcareous heath occurs on soil where there is no drift or foreign 

 soil of any kind ; and the transitional nature of the vegetation 

 is due to a secular and gradual change in the nature of the 

 primitive, calcareous soil. 



Again, many of the higher summits of that portion of north 

 Derbyshire which is indicated, on the geological maps, as 

 consisting of Carboniferous Limestone are capped by a layer 

 of non-calcareous chert ; and on this, of course, normal siliceous 

 grassland and moorland frequently occur. 



