294 SCIENCE PROGRESS. 



close to the granite often show little or no change that can 

 be certainly attributed to heat, and the rhyolite fragments 

 in a highly altered calcareous breccia retain all their minute 

 structures — cryptocrystalline, microspherulitic, and perlitic — 

 in perfection. Where, however, the rocks before meta- 

 morphism contained decomposition products, these have 

 given rise to new minerals, the characteristic green pinitoid 

 substance being converted to a mixture of white and brown 

 micas. In the acid tuffs, too, which were more composite 

 and more weathered than the lavas, various metamorphic 

 minerals have been set up. 



The augite-andesites of the same district had been ex- 

 tensively weathered prior to metamorphism. Chloritic 

 minerals, calcite, chalcedony, and quartz had been formed 

 from the pyroxene and felspar, and were partly disseminated 

 through the mass, but largely collected in little veins and in 

 the abundant vesicles of the rocks. These weathering pro- 

 ducts were the parts most readily affected by heat. The 

 chloritic mineral has been converted into biotite, or, where 

 it was associated with calcite, into green hornblende, which 

 specially characterises the metamorphosed amygdules : 

 chalcedonic silica has been transformed into crystalline 

 quartz. Nearer the granite the lavas are more altered, 

 sphene, magnetite, pyrites, etc., being formed ; the por- 

 phyritic felspars are replaced by a mosaic of new felspar 

 substance ; and finally the whole mass of the rock is recon- 

 stituted into a fine-textured ground. 



A more basic type of lava shows similar phenomena, 

 but, in consequence of the higher percentage of lime pre- 

 sent, the new-formed minerals are not quite the same. 

 Green hornblende here predominates over biotite, and 

 an augite has also been formed, especially in the amygdules 

 and veins. Another characteristic mineral is epidote. 

 Some large amygdules show a complex association of lime- 

 bearing and other metamorphic minerals — garnet, horn- 

 blende and actinolite, augite, quartz, sphene — and, in the 

 centre of the largest ones, some residual calcite recrystal- 

 lised but not decomposed. Another type of basic lava — 

 the well-known hypersthene-bearing rock of Eycott — is 



