576 THE JOURNAL OF GEOLOGY. 



elimination of the carbonic acid, while pure limestones or dolo- 

 mites, under the same conditions, merely recrystallize without 

 chemical change. In other words, the carbonates -are decom- 

 posed in thermal metamorphism only in the presence of silica in 

 some available form to take the place of the carbonic acid. 

 Interesting illustrations of this are given by some of the rocks 

 which have come under our notice. r The Strap granite in West- 

 moreland metamorphoses certain basic lavas containing amygdules 

 of various dimensions, many of which were occupied, prior to the 

 metamorphism, by calcite. Near the granite the smallest of 

 these calcite-amygdules are converted into various silicates rich 

 in lime, the silica having been derived from decomposition-pro- 

 ducts lining the original vesicles or from the immediately adja- 

 cent portion of the rock. In the larger metamorphosed amyg- 

 dules, on the other hand, only the outer layers are transformed 

 into lime-silicates, the interior still consisting of calcite ; which, 

 however, has recrystallized during the metamorphism, as is 

 proved by its moulding the silicates and being penetrated by 

 needles of actinolite, etc. Analogous appearances characterize 

 veins and lenticles of calcite in shales and the converse case of 

 argillaceous nodules imbedded in pure limestones and dolomites. 

 The conclusion is that carbonic acid is displaced from the calcite 

 only when there is in the immediate neighborhood either free 

 silica or some substance capable of furnishing silica. Where cal- 

 cite and quartz have recrystallized side by side in a meta- 

 morphosed rock, they are always separated by some one or more 

 lime-bearing silicates, but their distance apart may be very small, 

 and we deduce that the migration of silica to take the place of 

 carbonic acid has been restricted to extremely narrow limits. In 

 some highly altered rocks the distance is not more than one- 

 twentieth of an inch. 



The limit of migration of material no doubt increases with 

 the temperature of metamorphism. This is well illustrated by- 

 some calcareous ashes or tuffs. At a considerable distance — say 

 a thousand yards — from a large granite intrusion, the carbonic 

 'See especially Quart. Journ. Geol. Soc. (1893) vol. xlix., pp. 359^371- 



