206 



H. Dewey 8f Dr. J. S. Flett— 



100-86 



99-51 



100-25 



100-14 



I. West side of Tayvallich Peninsula, Argyllshire (analysed by E. G. Radley, 

 S. 12453). 

 II. Gatelochside Burn, near New Cumnock, Ayrshire (analysed by J. J. H. 

 Teall). "The Silurian Rocks of Britain" ; vol. i, Scotland: Mem. 

 Geol. Surv., 1899, p. 85. 



III. Tregedden, South Cornwall (analysed by E. G. Radley, E. 5791). 



IV. Devonport Workhouse Quarry (analysed by W. Pollard). " The Geology 



of the Country around Plymouth and Liskeard " : Mem. Geol. Surv., 

 1907, p. 97. 



The Albite-diabases. 



With the spilites both in Europe and America, diabases invariably 

 •occur, representing the intrusive magma that consolidated below the 

 surface. Some of them contained olivine, but their characteristic 

 minerals are purplish augite (of various shades), plagioclase felspar, 

 and titaniferous iron-ores. Like the spilites they are rarely fresh ; 

 their olivine is always decomposed and their augite is frequently 

 replaced by chlorite, epidote, and calcite, though it is far more 

 commonly in good preservation than the pyroxene of the spilites. 

 The only felspar in the vast majority of these rocks is albite, and it is 

 often present in surprising quantity. That basic plagioclase originally 

 formed the inner zones of many felspar crystals may be assumed from 

 the frequency with which the albite has a spongy centre filled 

 with epidote, prehnite, and chlorite, while the margins are compact. 

 But in these diabases veins and segregations occur that are of paler 

 colour and especially rich in soda-felspar, in fact they are almost 

 pure albite rocks. Instances may be cited from Newlyn Quarry, 

 Cornwall (14), and Trusham in Devon, and similar rocks have been 

 described from Germany by Brauns (15). On analysis many of the 



1 A substance is reported as not found (nt. fd.) when, though searched for in 

 the usual way, no traces of it have been obtained by the analyst. 



