RESEARCHES IN THERMAL METAMORPHISM. 293 



The earliest observations of thermal metamorphism in 

 igneous rocks were made by Allport (1876) on certain 

 diabases near the Land's End granite, and similar pheno- 

 mena have been described by Lossen in the Harz and by 

 other geologists in several other districts. The chief features 

 noted were the conversion of auoite to orreen hornblende, 

 and the recrystallisation of the felspar ; and these are found 

 everywhere to be highly characteristic changes. Brown 

 mica may be formed with, or instead of, hornblende, but, in 

 some cases at least, this seems to arise not directly from the 

 augite but from its chloritic decomposition products. Other 

 basic rocks show like transformations. The gabbro meta- 

 morphosed by the Carrock Fell granophyre in Cumberland 

 has developed green actinolitic hornblende and brown bio- 

 tite (20). The latter occurs chiefly around the grains of 

 iron ore, from which it has probably taken up some ferrous 

 oxide and titanic acid. At the same time the turbid felspars 

 have become clear. Callaway (21) has described the meta- 

 morphism of diorites in the Malvern range and in Galway. 

 In the former case the hornblende, perhaps previously 

 weathered, has given rise to deep brown biotite in the 

 vicinity of an intruded granite, while white mica has been 

 produced from the plagioclase felspar. In the other case 

 the plagioclase has been simply recrystallised, while a 

 chloritic mineral, epidote, and rarely biotite, represent the 

 original hornblende. In an altered diorite on the western 

 flanks of Dartmoor McMahon (22) finds a red mica as the 

 characteristic secondary mineral, most of the hornblende 

 having disappeared. 



On the metamorphism of volcanic rocks we have fuller 

 information. Besides the cases already noted in Saxony, 

 there are most instructive examples in the English Lake 

 district, especially around the Shap granite, where lavas 

 and tuffs of acid, intermediate, and basic composition may 

 be studied in various stages of alteration (3). One general 

 conclusion there brought out is confirmed elsewhere, viz., 

 that the acid rocks are commonly much less liable to meta- 

 morphism than the more basic. This is doubtless due to 

 their more simple chemical composition. The rhyolites 



