G. W. Robinson 379 



from them must be of comparatively recent origin. The sedimentary 

 rocks present a different problem. The shales of Cambrian, Ordovician 

 and Silurian age are among the oldest sediments of the Geological record. 

 While they are not metamorphic in the same sense that the Anglesey 

 rocks are metamorphic it must be recognised that they differ considerably 

 from the corresponding sediments of, say, Cretaceous age. Van Hise^, 

 who is disposed to denote all changes in the constitution of a rock as 

 metamorphic, distinguishes various zones of metamorphism. Immedi- 

 ately below the permanent water-table is the zone of cementation. 

 Lower still is the zone of regional metamorphism (anamorphism). 

 Considering the enormous thickness of the J*alseozoic sediments of 

 North Wales, it will be seen that regional metamorphism must have 

 played an important part in determining the present structure and 

 composition of these rocks, so that although they may originally 

 have consisted of mud and clay, their subsequent treatment has 

 largely led to the decomposition of whatever colloid materials were 

 present. 



Another theory as to soil plasticity is that represented by Atter- 

 berg^. This theory supposes that plasticity is due to the presence of 

 minute flake-like (schiippenformig) particles. Atterberg mentions cer- 

 tain minerals which can, when in a fine state of division, display plas- 

 ticity. Among these are kaolinite, talc, serpentine, biotite, muscovite, 

 limonite and haematite. The relation of the mineralogical composition 

 of the North Wales rocks to the plasticity of their resultant soils can 

 only be settled by a minute investigation. Until such an investigation 

 has been made it is scarcely safe to decide on Atterberg's theory or 

 its application to the area in question. 



2. Sedentary soils and soils derived directly from local drift 

 deposits contain remarkably high proportions of fine gravel, This is 

 particularly the case in the subsoils. One sedentary soil in Carnarvon- 

 shire contained over 40 % of fine gravel in the subsoil. In the soils 

 hitherto examined in other parts of Britain the fine gravel is usually 

 by far the smallest fraction except in the case of sands which contain 

 very little clay. 



3. Microscopic examination of the fractions obtained in mechanical 

 analysis shows that the coarser fractions are mainly formed of un- 

 decomposed parent rock. The sand fractions are quite different in 



' Van Hise, Treatise on Metamorphism, U.S. Geol. Survey Monograph, XLvn. 

 ^ A. Atterberg, Int. Mitt, fiir Bodenkunde, vol. ni, p. 1. 



