18 ME. H. C. SAEGE^T OX LOWEE CAEBONIFEEOUS [vol. Ixxili, 



latter often said to contain radiolaria, is well known. Sir Archibald 

 Geikie ^ has pointed out this association in the case of pillow-lavas 

 in such widely scattered localities as Cader Idris, the Lizard, the 

 southern flanks of the Scottish Highlands, the North of Ireland, 

 Saxon}', and California. Mr, Dewe}" & Dr. Flett - have recorded 

 the constant association of the Cornish spilites with radiolarian 

 chei-ts, which they ascribe to the escape and decomposition of the 

 soda -silicates in the igneous rock, whereby silica would become 

 available for assimilation b}- the radiolarian plankton. 



The i^hysical conditions of the Carboniferous Limestone sea were 

 doubtless, in general, unfavom-able to the development of organisms 

 requiring silica for their tests ; but an analog-ue to the Cornish 

 cherts is to be found in the silicified limestone or quartz-rock, 

 formed by the replacement of carbonate of lime b}'' silica, which so 

 often occurs in Derbyshire in the areas of former volcanic activity. 

 These rocks have been described by Dr. Bemrose,^ and he attributes 

 their occurrence to the action of deep-seated thenual waters holding 

 silica in solution. The liberated silica, which w^ould seem to be a 

 necessary product of the intense alteration that the spilitic lavas 

 have undergone, would probably, unless it were borne away by 

 currents, contribute to this result. Volatilized silicates held in 

 juvenile gases, the later efforts of volcanic energy, would also 

 contribute abundant sihca, which would be available for the for- 

 mation of quartz-rock. Ste. -Claire Deville and other investigators 

 have shown that ' emanations of carbon dioxide mark the dying-out 

 of the volcanic energy,' * and it seems probable that the reactions 

 originated by these components of juvenile gases Avould effect the 

 metasomatism in question. 



Instances of the occuiTcnce of quartz-rock in the immediate 

 neighbourhood of vents may be seen at Bonsall, close to the Ember- 

 Lane vent ; and at Ashover directly over the tuff of that locality. 

 Near Bakewell, in close proximity to the Crackendale vent, there 

 are thick deposits of chert which may, almost without doubt, be 

 ascribed to similar agency. The same observation applies to the 

 chert which occurs so abundantly in the neighbom-hood of Castleton, 

 again in close proximity to the scene of volcanic activity. In the 

 limestone of the Crich inher there are thin pai-tings consisting 

 entirely of microscopic crystals of quartz. 



Evidence in support of the view that the metasomatism resultmg 

 in the Crackendale type of chert is due to agents of volcanic origin 

 may be found (apart from the fact that it appears to occur always 

 in the area of former volcanic activity) in the presence of streaks 

 and patches of crystalline calcite and fluorspar embedded in the 

 chert, and also in the development, in cavities, of quartz-crystal.s 



' 'Ancient Volcanoes of Great Britain ' vol. i (1897) p. 193. 



2 Geol. Mag. dec. 5, vol. viii (1911) p. 245. 



3 Q. J. G. S. vol.liv (1898) pp. 169-82. 



^ See F. W. Clarke : ' The Data of Geochemistry " Bull. U.S. Geol. Surv. 

 No. 616. 3rd ed. (1916) p. 262. 



