3 02 SCIENCE PROGRESS. 



daloidal lavas near the Shap granite are especially instruc- 

 tive (3), (32). These contain large amygdules, up to two 

 inches or more in length, which before metamorphism were 

 filled by decomposition products, including calcite, a chloritic 

 mineral, and quartz. The calcite seems in general to have 

 occupied the central part of the vesicle. As already men- 

 tioned, these substances have entered into new mineral 

 combinations, among which lime-bearing silicates figure 

 prominently ; but in the centre there remains some calcite 

 undecomposed. The manner in which this mineral is 

 penetrated by needles of actinolite, etc., proves that it must 

 have recrystallised during the metamorphism, and clearly 

 its temperature must have been the same as that of other 

 calcite in the same vesicle, which has been destroyed to 

 form garnet, augite, etc. We have already remarked that 

 in thermal metamorphism calcite is decomposed only in the 

 presence of available silica to replace the carbonic acid 

 driven off, and we now see that silica does not travel for 

 this purpose more than a very small fraction of an inch. 

 Results quite confirmatory of this extremely narrow limit to 

 the migration of material are obtained from a study of cal- 

 careous nodules in metamorphosed shales, of the "spots" 

 in altered slates, and of the dimensions of individual crystals 

 of various metamorphic minerals. Whatever part water 

 may play as a solvent in thermal metamorphism, it appar- 

 ently does not act to any appreciable extent as a carrier to 

 facilitate interchange of material between different parts of 

 a rock. 



In the opinion of the present writer the idea of solution 

 can only be entertained in this limited sense, the action of 

 any solvent or flux supposed to be involved being in the 

 strictest sense local. We have hitherto discussed no specu- 

 lations as to the processes by which new mineral combina- 

 tions are set up during thermal metamorphism, and crystals 

 of considerable size formed in the heart of what might be 

 regarded as a solid rock. The question is one to which 

 investigation has scarcely been directed by most inquirers. 

 Hutchings (9) has recently approached it, his point of view 

 being such as leads him to lay stress on the solvent action 



