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 Gallii), 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 



