30 MR W. F. P. M'LINTOCK ON THE ZEOLITES AND ASSOCIATED MINERALS 



niorphic changes which it produced. These changes resulted in the production of 

 the following minerals : — 



Prehnite. Garnet. 



Epidote. Sphene. 



Pyroxene. Albite. 



Hornblende. 

 With the exception of sphene, all of the above minerals occur as original constituents 

 of the cavities, and the result of the metamorphism has been to recrystallise them in 

 the reverse order to that in which they were originally deposited. Thus with the 

 lime-bearing silicates the metamorphic changes progress as follows: — scolecite -> 

 prehnite -> epidote -> garnet, whilst the original order of deposition was : — garnet -> 

 epidote ->- prehnite -> scolecite. Similarly with the lime-soda zeolites the order with 

 rising temperature is : — thomsonite -> albite and prehnite -> epidote -> garnet, whilst 

 the original order of deposition was : — albite -> epidote -> prehnite -> thomsonite. 



The pyroxene and hornblende have been developed from the chlorite and scolecite 

 or prehnite, whilst the sphene owes its origin to the titanium originally present in 

 the augite and iron oxides of the rock, which found its way into the epidote (see 

 analysis) and probably also into the chlorite of the vesicles. 



The above evidence confirms in a striking way the view that the vesicles were 

 originally filled under conditions of falling temperature, for, with the rising tempera- 

 ture occasioned by the intrusion of the granophyre, the only change has been to 

 obliterate the minerals formed originally during the last and lowest temperature 

 stages of the hydrothermal phase, and to form at their expense those minerals which 

 crystallised in the first and highest temperature stages of that phase. Not only so, 

 but the metamorphic minerals, considered from the point of view of their general 

 distribution, have been developed in the reverse order to that in which their original 

 representatives were laid down in the cavities. 



An interesting point arises from the abundance of garnet in the metamorphosed 

 amygdales and its rarity in the unaltered ones. This is probably to be ascribed to 

 the great difference between the pressures under which the two sets of minerals were 

 formed. Dr L. L. Fermor* has pointed "out that garnet is a high-pressure mineral., 

 and — as recrystallisation of the vesicle-minerals under the influence of the intrusion 

 must have taken place at a considerable depth and, consequently, at a considerable 

 pressure — the tendency would be for it to form at the expense of the other, less dense 

 lime-alumina silicates. In other words, the heat of the granophyre, combined with 

 the high pressure, was steadily making the vesicles less hydrous and more favourable 

 to the formation of garnet. Under the hydrothermal conditions, on the other hand, 

 the temperature was falling, the pressure was never great, and the vesicles were 

 becoming more hydrous and less favourable to the formation of garnet, or the pre- 

 servation of any that might have crystallised. To judge from the vestigial appear- 

 * L. L. Fermoe, Records (JcoL Surv. India, 11)13, vol. xliii, pt. 1, p. 42. 



