E. B. Bailey &f G. W. Gmhham — Plagioclase Felspars. 251 



albitized felspars in certain basalts of Cape Marsa, and we have 

 more recently noticed the same phenomenon in many eruptive rocks of 

 Scotland. 



Termier ascribes the production of albite, in the cases -which he has 

 studied, to the influence of percolating surface waters,^ which have 

 above all things leached out the lime from the felspars and other 

 minerals with which they have come in contact. Occasionally, where 

 for some reason the subsoil waters have become unusually rich in soda, 

 they have, according to Termier, been able to leave behind some 

 of this base in partial exchange for the lime removed. More generally, 

 however, the place of anorthite is taken by a silicate containing neither 

 lime nor soda, such as chlorite, and the albite still remaining is merely 

 a residue derived from the original plagioclase. Duparc & Pearce, 

 on the other hand, have not entered upon any theoretical discussion of 

 the processes involved in albitization, while our own experience has 

 led us to regard the change as probably belonging to the category of 

 juvenile reactions which igneous rocks undergo while still hot and 

 richly charged with volatile and soluble ingredients.^ 



Two quite distinct suites of rocks have been specially examined during 

 our inquiry into the rationale of albitization, namely, the Carboniferous 

 lavas and the Permo-Carboniferous intrusions of the Central Valley of 

 Scotland. 



Carboniferous Lavas. 



Albitization has been noticed among various basaltic and trachy- 

 doleritic lavas. It is local in its incidence, for scarcely an example is 

 known from East Lothian, while many occur among the lavas of 

 Arthur's Scat, Edinburgh. Again, in the Cathkin Hills, south of 

 Glasgow, few cases have been discovered, while to the north of Glasgow, 

 in the Campsie Fells and Kilpatrick Hills, every stage leading to com- 

 plete albitization can be studied (e.g., on the hill-top above Lennoxtown 

 and 150 yards north-east from Carbeth). 



21ie alteration is (juidecl to a notable extent lij the cleavage and other 

 cracks of the original fehpars, so that kernels and ragged patches of the 

 latter are often preserved in the heart of the secondary albite, and 

 enable us to judge of the composition of the portion which has been 

 replaced. The Campsie and Kilpatrick specimens in the Geological 

 Survey collection demonstrate perfectly clearly that the more basic the 

 felspar the more liable it is to be albitized. Thus, in a porphyritic 

 basalt, the phenocrysts are always attacked before the less basic 

 felspars forming the ground-mass, and, in fact, the latter are seldom 

 affected in the least degree until the former have been more or less 

 completely replaced. Not only this, but in a given felspar the more 

 basic portions are picked out first, so that even a narrow, alternating 

 zone of more basic composition is replaced, in part at least, before the 

 more acid portions on both sides (PI. X, Fig. 2) ; it is, for this 



' Although not bearing directly upon the point at issue there is much suggestive 

 matter in Sullivan's "Interaction between Minerals and Water Solutions", Bull. 

 U.S. Geol. Sun-., 1907, No. 312. 



2 It must be understood that, even if we are correct in ascribing the greater part of 

 the albitization studied by ourselves to juvenile changes, this does not preclude the 

 possibility of Termier's explanation holding good in other districts. 



