Iv] GRASSLAND ASSOCIATIONS 117 
The soil of the calcareous grassland of the limestone slopes is 
shallow, sometimes not even an inch (about 2°5 cms.) deep. In 
colour, it varies from a whitish grey when the lime-content is 
very high, to brownish or even reddish-brown when the lime- 
content is lower and the iron-content higher. At the foot of a 
cliff, the soil may be a metre or more in depth: commonly it is 
about a sixth to a third of a metre deep. The soil of lime- 
stones is usually described as being very dry and porous; but it 
is only the newer and whiter soil of which this may correctly 
be stated. The older and darker soil is of a marly nature, and 
is neither specially dry nor specially porous. Similarly, the 
newer and yellower soil of the sandstones may be dry and 
porous, whilst the older and blacker soil of the sandstones is 
retentive of water. Analyses prove that the range of variation 
of water-content of the soils derived from the limestone rocks 
is roughly paralleled by that of the soils of the sandstones and 
shales. Just as the water-content of the non-calcareous soils 
varies directly as the humus-content, so the water-content of 
the limestone soils varies inversely as the lime-content. The 
former result is doubtless due to the water-absorbing properties 
of humus: the latter seems to be due to the fact that calcium 
carbonate is dissolved by water containing carbon dioxide; and 
thus as limestone soils become older they lose more and more 
lime and acidic humus then tends to accumulate. Marshy 
places occur on the limestone just as they do on the sandstones 
and shales. However, it may be said that, in general, such 
localities are least frequent on the limestones, more frequent 
on the sandstones, and very numerous on the shales. The 
marshy places on the limestones bear a very different flora from 
those of the sandstones and shales, just as the dry limestone 
soils possess a very different flora from such soils on the sand- 
stones; and it thus appears to be quite impossible to explain 
the distribution of the humus-loving and the lime-loving 
species respectively by any relations of the water-content. 
The abundance of the bracken (Pteris aquilina), the gorse 
(Ulex Galliz), and the rush (Juncus effusus), which is so very 
noticeable a feature of the various types of siliceous grassland, 
is not seen on the calcareous grassland of this district. In fact, 
on the limestone slopes below 1000 feet (305 m.), these plants 
are absent or rare; and even on the more or less leached soils 
