British Pillow-lavas. 207 



diabases differ but little from ordinary diabases except in tbe slightly 

 higher percentage of soda that they contain. Others, however, 

 contain as much as 5 per cent, of soda, an unusual amount for 

 rocks of this type. There can be no doubt that they are a basic 

 group characterized by abundant alkalies and hence possessing 

 affinities with essexite, as has been recognized by Elett (16), 

 Ehrmannsdorfer (17), and Weber (18). 



It is equally certain that in some cases they have been albitized 

 and have had. additional soda-felspar introduced after they had 

 consolidated. An example occurs in the Kit Hill diabase (19). 

 It has large rectangular felspars with interstitial anidiomorphic augite 

 and iron-ores. The two last-named minerals are rather decomposed, 

 but the felspar is very fresh, clear, and homogeneous. It occurs 

 in fairly large crystals which are not zoned, and it has the optical 

 properties of albite. By means of bromoform it was separated from 

 the crushed powder of the rock, and analyses confirmed the optical 

 tests by showing that it was almost chemically pure albite. 



In Cornwall ami Devon the albite-diabases, like the spilites, were 

 completely decomposed and albitized before the late Carboniferous 

 earth-movements, and were hornfelsed by the granites. Their soda- 

 felspar was then replaced by andesine and labradorite, which is still in 

 excellent preservation. Here again it is certain that the albitization 

 ensued very soon after the rocks were intruded, and we infer that it 

 was of pneumatolytic character. 



MlNVERITES (PrOTEROBASEs). 



Certain rocks from North and East Cornwall that were described 

 by Teall (20) as hornblendic diabases were subsequently described by 

 Flett (21) as proterobases. In 1910 Dewey (22) proposed the name 

 minverite for similar rocks from St. Minver, North Cornwall. Thev 

 are much like the diabases, but contain fresh brown primary horn- 

 blende. Sometimes traces of olivine (never fresh) occur in them ; 

 they are not usually ophitic, and contain much brown and purplish 

 augite. Primary alkali-felspar, mostly albite, is often abundant in 

 the minverites. Analyses of these rocks from Cartuther and 

 St. Mabyn show that they are of the sub-alkalic group of basic rocks, 

 very close in many respects to the essexites. Often they contained 

 a limited amount of lime-soda felspar, now replaced by albite, epidote, 

 and prehnite, or in some cases almost wholly albitized. Other 

 features of this group are the striking abundance of apatite in long 

 thin prisms, and the occasional presence of deep-brown biotite. At 

 Polyphant they are closely associated with the hornblende picrites. 



Q.UARTZ-DIABASES. 



Here and there in Cornwall quartz -diabases (23) occur among the 

 spilites and albite-diabases, and belong to the same volcanic epoch. 

 Yet in these rocks the basic felspars remain, every zone is perfectly 

 distinct, the most basic in the interior, the most acid externally. 

 Clearly, then, the albitization is a process intimately connected with 

 the nature of the individual rock; the spilites and quartz-free diabases 

 show it, but the quartz -diabases do not. We are reminded of the manner 

 in. which some granites are attended by pneumatolytic phenomena 



