Iv] GRASSLAND ASSOCIATIONS 125 
the former one. When, for example, a sheet of open water 
becomes filled up with silt and peat, the aquatic formation has 
succumbed and a fen or a peat moor has taken its place. There 
are, then, not only intermediate associations in any single plant 
formation, but also passage associations leading from one 
formation to another. The limestone heath is such a passage 
association. Geologists have long termed certain strata between 
two geological formations transitional or passage beds; and it 
is to be expected from the nature of the case, that similar tran- 
sitional tracts of vegetation should connect certain allied plant 
formations. Doubtless some difference of opinion may arise 
as to which of two plant formations a particular passage 
association should be referred; but such a matter is not really 
one of fundamental importance. 
Some of the bare or almost bare limestone rocks at altitudes 
approaching 1500 feet (457 m.) also furnish an interesting 
mixture of lime-loving and humus-loving plants. For example, 
the following mixture of lime-loving and humus-loving spécies 
was noted at Thirkelow rocks, south of Buxton :-— 
Asplenium viride Galium saxatile 
Poterium Sanguisorba Deschampsia flexuosa 
Sedum acre Festuca ovina 
Vaccinium Mprtillus Nardus stricta 
Calluna vulgaris Luzula erecta 
Thymus Serpyllum (agg.) 
In the case of the heather and the bilberry, it was found 
that, whilst some of the roots of the plants were closely 
appressed to the bare limestone rock, other roots of the same 
plant were embedded in black humus formed chiefly of decaying 
lichens and mosses. It is a matter for experiment whether 
seedlings of these plants will germinate and arrive at maturity 
if grown in a calcareous soil destitute of humus. 
The following list of species are illustrative of the limestone 
heaths of north Derbyshire :— 
