466 Notices of Memoirs — W. G. Fearnsides — W. Boyd Dawkins. 



the Afon Arddu (whose delta parts the two lakes of Llanberis) is 

 exceptional, and is very rich in well-formed trigonal prisms of blue 

 tourmaline. The sand from the Afon Hwch, its tributary from 

 between Moel Eilio and Moel Goch, is even more surprising, and 

 in the sand from the spits along the flatter reaches of this burn 

 tourmaline can generally be distinguished with a pocket lens. 



I have therefore mapped the Cwm Dwythwc on the 6 inch scale, 

 and in mapping have found the tourmaline rocks in situ. They are 

 mostly coarse grits, grits, flags, and slaty flags, and occur along the 

 horizon of the unconformity between Cambrian and Ordovician rocks. 

 The tourmaline is not clastic, but has been formed in situ fi'om the 

 felspathic ground-paste of the grits or flags, and clustered new- 

 formed needles enter and pierce the quartz pebbles of the grit or the 

 chloritoid ground-mass of the slate in a most fascinating manner. 

 There has been thrust-faulting along the unconformity, but no large 

 intrusive mass of igneous rock has been observed within five miles of 

 the locality. Tourmaline new-formed in the slate and the remains of 

 tuning-fork graptolites can be found within 3 or 4 inches of each 

 other. The tourmaline is a soda-bearing variety. 



VII. — The Derivation of Sand and Clay from Granite.^ By 

 Professor W. Boyd Dawkins, D.Sc, F.R.S. 



rilHE decomposition of granite by the attack of carbonic acid in the 

 I rain-water on the soluble crystalline constituents of the granite 

 results in the formation of a surface covering more or less complete over 

 the solid rock which can only be studied in non- glaciated regions. It 

 is conspicuous by its absence from the ice-swept granite areas of the 

 Lake country, of Scotland, and of Ireland, and of Middle and Northern 

 Europe. 



The quartz in the granite has resisted decomposition, and where the 

 finer products of decomposition have been swept away it forms a coarse 

 sand, each grain presenting an irregular surface indented by the 

 felspars and micas as they cooled from the heated magma. These are 

 traceable more or less through a large number of sandstones, and more 

 especially through those of the Millstone Grits and Coal-measures of 

 Middle and Northern England. 



The attack of the rain-water containing carbonic acid on the micas 

 results in the decomposition of the biotite and to a lesser degree of the 

 muscovite, while the soluble felspars, such as orthoclase, are com- 

 pletely dissolved, constituting hydrated silicates of alumina and new 

 minerals such as kaolinite and secondary minutely crystalline muscovite. 

 All these occur in the china clay of Cornwall and Devon, and are 

 invariably associated with grains of quartz, primary mica, and tourma- 

 line present in the unaltered granite. These constituents occur in 

 all the samples ranging from the purest china clay through the whole 

 series which have as yet been examined, with the addition of others of 

 local derivation. 



' Read before the British Association, Section C (Geology), Dublin, Sept. 1908. 



