248 Prof. T. G. Bonney — Foliation and Metamorphism. 



One or two silicates, larger in size than the other constituents, are 

 sometimes formed in this stage of metamorphism, but as they may 

 become very impure by including material from the ground-mass, 

 precise identification is often difficult, but they probably represent 

 dipyr, couseranite, and a colourless chloritoid. In rare instances 

 small idiomorphic garnets are found in rocks, the matrix of which 

 shows little signs of change. These have a peculiar structure which 

 distinguishes them from the ordinary garnet of true schists and 

 igneous rocks, and are sometimes associated with "bunches" of 

 small actinolitic hornblende, almost colourless. The evidence as to 

 the origin of these minerals is not decisive, though it suggests the 

 action of heated vapours (42). 



The knoten and prismen, associated with crinoid and other organic 

 relics of Liassic age, in the mountains above the Lukmanier Pass, 

 on the eastern slope of the Nufenen Pass, and on or near the Nufenen 

 Stock, are remarkable instances (41). 



I have passed over one mineral, tourmaline, which is generally, if 

 not always, a result of metamorphic action, because its mode of 

 occurrence links it more closely with contact metamorphism. It 

 is apparently the result of pneumatolytic action on the aluminous 

 minerals in a rock, the felspars and the biotites in those of igneous 

 origin (usually the granites) and the clayey constituents of the 

 sedimentary. It also occurs in veins traversing these rocks, and 

 especially granite masses, in association with quartz, fluor-spar, 

 lithia-micas, chlorites, cassiterite, and one or two other minerals of 

 less frequency, and the main difference from the ordinary results 

 of contact metamorphism, which I have not discussed because they 

 form so distinct a category, is that, as the tourmaline appears in the 

 intrusive rock, which has produced that metamorphism, this mineral 

 must be later in date than it. 1 Of this mode of occurrence the rock 

 called luxulyanite is the most striking example (43). 



The conversion of the felspar in a mass of granite into the material 

 called china clay, so strikingly illustrated in the Devon-Cornwall 

 region, and often closely associated with the tourmalinizing agencies, 

 is strictly speaking a metamorphic process, and a still more striking 

 change is occasionally found in the replacement of the quartz in 

 a granite by fluor-spar (44). 



Other igneous rocks have undergone conspicuous metamorphism 

 since they first solidified. Of these the replacement of augite by 

 hornblende, and felspar by saussuritic minerals, has already been 

 mentioned, but some diabases exhibit still greater changes. These, 

 however, so far as I have seen, occur only in small masses, and 

 generally in regions where pressure appears to have co-operated with 

 water. The result has been the removal of much Si0 3 and MgO, 

 and the formation of a peculiar chlorite with a much higher 

 percentage of alumina than in those ordinarily described (45). 



But serpentine is the most conspicuous instance of an igneous 



1 It may also appear in the associated sedimentary rock, which shows the 

 ordinary effects of contact metamorphism, but suggests by its mode of 

 occurrence that it is due to some subsequent action. 



