PROSPECTS AND POSSIBILITIES OF FORESTRY 43 



in the case of a self-sown crop on otherwise worthless ground, 

 when growth of any kind which pays for cutting must be 

 considered profitable. Scots fir will exist on sandstone, 

 beech on chalk, and even oak in the crevices of slate cliff, 

 but for a profitable growth to be made, a certain depth of 

 soil or porous stratum is absolutely essential, and the depth 

 necessary to maintain a given rate of growth will vary, to a 

 certain extent, with the supply of soluble plant food which 

 the porous stratum contains. A thin but rich soil will 

 sustain more growth than a thin and poor soil, and in this 

 respect the geological formation may be of importance, for 

 any soil is simply powdered rock. When we know that the 

 rock from which a soil or superincumbent stratum has been 

 formed is rich in mineral plant food, such as potash or 

 phosphoric acid, we may infer that the stratum itself is of 

 superior quality to one formed of almost pure quartz or 

 carbonate of lime. Greensand or granite soils, for instance, 

 are richer in mineral food than those from millstone grit or 

 chalk, and soils formed from the former are usually more 

 productive than those from the latter. But, even here, 

 moisture or mechanical condition may affect fertility, and a 

 soil which ought theoretically to be fertile may be actually 

 poorer than one which might be supposed to be poor. 



But, as a general rule, a knowledge of the geological 

 formation, the depth of porous stratum, and the average 

 rainfall, will enable one to form, as already stated, a fairly 

 accurate estimate of the value for timber production of any 

 large tract of country which cannot be utilised for agri- 

 cultural purposes except at nominal grazing rents. 



The gravels and sands of the Tertiary Period, the poor 

 and stony soils on the G-reensand, the lower slopes of hill 

 ranges formed by the Mountain Limestone, Millstone Grit, 

 Silurian and Metamorphic rocks, etc., present extensive 

 tracts which are capable of growing profitable crops of 

 timber, evidence of which is found in plantations already 

 existing on them. As a general rule, the more finely a 

 rock is laminated, and the more easily it weathers down, 

 the more valuable it becomes for timber growing ; for the 

 roots of trees are able, in themselves, to force their way into 

 stratified rock, and secure both food and foothold when once 



