24 MR W. F. T. M'LINTOOK ON THE ZEOLITES AND ASSOCIATED MINERALS 



against it. Albite was in many cases the earliest vesicle-mineral, for it occurs in 

 chlorite and epidote, against which it shows usually sharp, idiomorphic outlines. 

 The succession, albite, epidote, prehnite, scolecite, is proved in many cases, and 

 illustrates very well what happens during deposition under hydrothermal conditions. 

 The albite was deposited first and is enclosed in places by the epidote which replaces 

 it but slightly ; with falling temperature prehnite is formed, which encloses and 

 frequently corrodes and replaces the albite, a phenomenon also observed by Fenner,* 

 who figures an example exactly similar to many of those seen in the present instance. 

 Finally came the scolecite phase, during which the albite was also replaced, - the 

 epidote corroded, and even the prehnite attacked. 



The survival of much of the pegmatitic augite raises a difficulty, for, although it 

 exhibits signs of corrosion and occasional marginal alteration to hornblende and 

 epidote, typically it is fresh and contrasts somewhat forcibly with the decomposed 

 condition of a good deal of the augite of the rock. The explanation is probably due 

 to the fact that when the solutions welled into the cavities they were saturated with 

 chlorite and silicates of lime and soda, and therefore incapable of attacking the 

 pyroxene. This is supported by the fact that in the subsequent phases albite is the 

 mineral which shows in the highest degree the phenomenon of corrosion and replace- 

 ment, the reason being that, after it had formed, it was constantly under the action 

 of lime-bearing solutions which were able to decompose it. 



The evidence that the vesicles were filled during the last stages of cooling may 

 be stated shortly as follows : — 



1. The vesicles sometimes contain pyrogenetic minerals. 



2. The mineral association as far as the evidence goes is one demanding a 

 temperature above the normal. 



3. The silicates (with the exception of chlorite) were deposited in order of 

 increasing hydration : — 



(a) Albite . Na 2 . A1 2 3 . 6Si0 2 



(b) Epidote . 4CaO . 3A1 2 3 . 6Si0 2 . H 2 ; H 2 = 1'98 per cent. 



(c) Prehnite . 2CaO . 3A1 2 3 . 3Si0 2 . H 2 ; H 2 = 4"4 per cent. 



(d) Scolecite . CaO . A1 2 3 . 3Si0 2 . 3H 2 ; H 2 = 13"8 per cent. 



4. There is direct evidence that albite, epidote, and prehnite in the order named 

 each passed through a phase of stability when they were deposited, and that they 

 were more or less unstable to the succeeding phases. 



5. Three periods, which grade into one another, can sometimes be recognised : — 



(a) Period of magmatic consolidation. 



(b) Period of pegmatitic crystallisation. 



(c) Period of infilling of the vesicles. 



The last-mentioned of the five points suggests analogies with the three phases 



* Lot: tit., p. 126. 



