MBTAMORPHIC LLMESTo.NKS TILLEY. 241 



minerals, bringing about a partial dedoloniitisation. The presence of fluorine is 

 established by the occurrence of chondrodite. The halogen is doubtless derived from 

 the igneous magma to which the metamorphism of these rocks is due. 



In the second class of marbles, however, whilst dolomite is present with the 

 calcite, the dominant silicate mineral is neither forsterite nor diopside, but tremolite. 

 The habit of crystalline schists which these rocks possess suggests that in their 

 metamorphism the element of shearing stress was dominant. Under these conditions 

 the antistress mineral, forsterite, does not develop. 



In the diopside-tremolite-marbles, dedoloniitisation has been complete, and the 

 residual carbonate mineral is calcite. With the disappearance of dolomite, the entry 

 of magnesia-free silicate is permitted, and in accordance with this, these rocks develop 

 subordinate plagioclase. The composition and texture of the rocks of this class allows 

 them to be regarded as contact types. 



In class (4) the pyroxene-garnet-marbles, an abundance of detrital constituents 

 has given rise to a varied assemblage of metamorphic minerals. Microcline arises 

 from detrital sericite, the excess alumina being absorbed in the accompanying silicates, 

 and scapolite develops in place of plagioclase, when the necessary volatile elements are 

 present. 



The pyroxene, garnet and epidote, show by their optical properties that a not 

 inconsiderable amount of iron is present in their molecules. The source of this iron 

 oxide, is doubtless ferrous carbonate or hydrated iron oxide (limonite), in the original 

 sediment. The hedenbergitic character of the pyroxene shows that portion of the 

 magnesia is replaced by FeO. 



The coloured garnet must be regarded as a grossular andradite solid solution. 

 This is confirmed by its paragenesis, and optically by the refringence. A possible 

 type of reaction which has led to the development of this mineral may be represented 

 as follows : 



6CaC0 3 + H 4 Al 2 Si 2 () 9 + Fe 2 a + 4Si0 2 = 2Ca 8 AlFeSi 3 O u + 6C0 2 + 2H a O. 



Epidote must be regarded as the youngest of the silicates. The presence of 

 the epidote-quartz symplektites around the garnet suggests that the development of 

 epidote in these cases is attributable to a degradation of the garnet molecule in a later 

 stage of decreasing metamorphism, 



3Ca :i (AlFe) 2 Si 3 12 + H 2 = 2HCa 2 (AlFe) :s Si :! O l3 + 3SiO 2 + 5CaO., 



the lime with carbon dioxide giving the associated calcite. A development from the 

 anorthite member of the plagioclase has also played a part. 



In the rocks of class (5) epidote is an abundant constituent, and must be given a 

 place as a primary constituent, derived either from a reaction of carbonates and ferric 

 oxide, with kaolin or sericite, in the latter case being accompanied by microcline. The 

 rocks of this class resemble those of class (4) with the distinction that garnet is not 

 represented and its place is taken by epidote. 



