378 Studies on the Palaeozoic Soils of North Wales 



over the analyses it will be seen that the clay fraction rarely exceeds 

 10 % in the case of soils derived from the shale or 7 % in the case of the 

 Anglesey and Carnarvonshire loams. Usually the clay fraction falls 

 considerably below these figures. The silt fractions, on the other hand, 

 particularly in the Palaeozoic silt loam, form a considerable proportion 

 of the soil. 



These circumstances, together with the high proportion of organic 

 matter, render the North Welsh soils easy to work where water con- 

 ditions are not unfavourable. A number of determinations were ijiade 

 of the plasticity-numbers of various typical soils according to the 

 method of Atterberg^. The plasticity-numbers for shale soils varied 

 from nothing to 10. In the case of the Anglesey medium loams no 

 appreciable plasticity was found. The heaviest of the Carnarvonshire 

 stony loams gave a value 7-2. According to Atterberg's standards, 

 these soils cannot be reckoned heavier than medium loams. 



The high proportions of silt and fine silt render soils sticky when 

 wet, but on drying they are generally quite friable. In order to deter- 

 mine the effect of fine silt on the plastic properties of a soil a glacial 

 clay from the vale of Clwyd, containing 34-4 % fine silt and 14-9 % 

 clay, was taken and the clay removed by sedimentation. The residue 

 was dried and the following plasticity numbers were obtained by the 

 Atterberg method : 



D 19 B D 19 B (clay removed) D 19 B (clay fraction) 



23-9 31-3 



It is of course probable that the high proportions of organic matter 

 contribute to render soils friable on drying. 



With regard to the plasticity of soils and the factors producing it, 

 two main theories may be noted. On the one hand there is the theory 

 that plasticity is due to the presence in soils of hydrated colloidal pro- 

 ducts. This theory has been recently discussed by P. Eohland^. If 

 this theory be correct the general absence of plasticity in the Palaeozoic 

 soils may be explained. The soils examined in this paper are derived 

 from three main types of rocks, namely, the igneous and volcanic rocks 

 of Carnarvonshire, the metamorphic rocks of Anglesey and the sedi- 

 mentary rocks found all over the area. From the nature of the first 

 two rock types they cannot be expected to contain pre-formed colloid 

 materials and whatever colloid clay material is found in the soils derived 



1 A. Atterberg, Int. Mitt, fiir Bodenkunde, vol. i, p. 10. 

 - P. Rohland, Die Tone, Vienna, 1910. 



