244 H. Dewey Sf Br. J. S. fflett— 



admitted by petrographers for many years. For a summary of the 

 literature on this subject we may refer to lloth (41). 



In its microscopic structures the adinole is no less evidently a re- 

 crystallized sediment. The lamination, cleavage, and bedding are 

 soon lost, and a cryptocrystalline or finely crystalline rock is produced 

 that has the closest resemblance to a chert. In North Cornwall and 

 Devon cherts and adinoles constantly occur together in the same 

 quarry. Very often they can hardly be distinguished except for the 

 fact that the felspar of the adinole has a lower refractive index than 

 balsam, while the quartz has the same or slightly higher ; moreover, 

 the adinoles are fusible -while the cherts are not. By further re- 

 crystallization the adinoles become a mosaic of albite grains that have 

 very closely the same mode of aggregation as the silicified shales 

 found associated with some mineral veins (such, for example, as the 

 flinty Ordovician shales of Parys Mountain in Anglesey). Now the 

 cherts are admittedly rocks that have been recrystallized from colloid 

 organic silica through the agency of aqueous solutions, and an allied 

 mode of origin seems probable for the adinoles. In the latter we may 

 often note the meshwork of veins filled with albite slightly more 

 crystalline than the matrix, through which the solutions were intro- 

 duced. The adinoles consist of quartz, albite, and chlorite (with 

 rutile and iron oxides), and these are also the minerals that most 

 characterize the post-volcanic or pneumatolytic stage of crystallization 

 in the albite-diabases. 



That adinoles should accompany albite-diabases is exactly what 

 might be expected, for the pneumatolytic or post-volcanic vapours by 

 which the intrusive rocks Avere albitized provide also the necessary 

 constituents for the albitization of the sediments. In this respect the 

 adiuoles are not unlike the tourmalinized killas that occurs at the 

 margins of the Cornish granites, where the same vapours as attacked 

 the sediments also attacked the granite itself, converting it into schorl 

 rock. The remarkable chemical changes that take place in the trans- 

 formation of shale into adinole are the clearest evidence that could be 

 adduced to show that vapours or solutions rich in soda were emitted 

 by the igneous rocks during the later stages of their crystallization. 



The Cheets. 



That pillow-lavas are usually accompanied by radiol'arian cherts is 

 now a well-established fact. Attention has been directed to this 

 paragenesis by Teall (42), and in all parts of Britain in several 

 geological epochs it has since been proved to hold good. 



As already stated, the spilitic rocks occur typically as submarine 

 lavas in regions remote from shore-lines where the water is of con- 

 siderable depth. These also are in a high degree the conditions that 

 seem favourable for the deposition of siliceous organic deposits. The 

 association then is a natural one, just as radiolarian cherts occur with 

 fine graphitic shales and limestones. 



But this is not the whole truth. How often do we find the cherts 

 nestling in the hollows between the pillows even where limestones 

 are few or absent, and where the sediments are of rather coarse grain 

 and belong to shallow- water facies. In Cornwall pillow -lavas are 



