35 



auto-metamorphism, the alteration of the pyroxene having 

 taken place as a result of the activity of residual magmatic 

 waters. 



A curious rock is that forming the first dyke between 

 Rosetta Head and King Point. Under the microscope it is 

 seen to consist of numerous little sheaf -like bundles of amphi- 

 bole ( ? actinolite) needles set in a clear and colourless micro- 

 crystalline matrix which high magnification reveals as a regu- 

 lar mosaic of interlocking quartz and plagioclase granules. 

 The felspar is evidently pretty basic, its R.I. being distinctly 

 greater than that of quartz, and it is apparently the predom- 

 inant colourless mineral, although relative proportions could 

 not be estimated. There is a fair sprinkling of ilmenite and 

 a little pyrites. Dark-green pleochroic spots in the amphibole 

 represent haloes round zircons. 



The rock may be termed a dolerite hornf els ; it appears to 

 be a dyke rock which has been completely recrystallized, but if 

 it belongs to the same suite as the other basic dykes the 

 different course of metamorphism is difficult to understand. 

 Most probably, however, this particular rock is older than 

 the granite, and this suggestion is strengthened by the record 

 of an axinite amphibolite from Rosetta Head by J. C. Moul- 

 den,(26) which, as he points out, has probably resulted from the 

 contact metamorphism of a basic igneous rock by the granite. 

 The present writer was not fortunate enough to encounter this 

 axinite-bearing rock in the field. 



V. Petrogeny. 



(1) GENETIC RELATIONSHIPS OF THE ROCKS. 



There is every reason to believe that a close genetic 

 relationship exists between all the different rock types. The 

 field evidence points to it, and is to a certain extent supported 

 by the mineralogical and chemical characters of the rocks. 



A variation diagram (fig. 4) has been constructed to 

 exhibit the chemical relations of the diorite, normal granite, 

 and even-grained granite; in this specific gravities have been 

 used as abscissae, and the continuity of the series is well shown. 

 In particular the practically rectilinear nature of the curves 

 for iron and magnesia (contained in biotite and the iron ores) 

 and for soda emphasises the regular withdrawal of these con- 

 stituents from the magma. 



A regards the albite syenite, while there is no direct 

 evidence that it is comagmatic with the other types, this is 

 strongly suggested by its ultimate field association with the 



(26) J. C. Moulden: Joe. cit. 



