194 REV. J. MILNE CURRAN. 



( Holocrystalline }' Diorites. 



Intermediate. a ( Diabase. 



[Crystalline with a paste ( Augite porphyrite. 



( Diabase porphyrite. 



Glassy j Taxih 7 l J te ' 



( C t 11* hi Basalt vitrophyr. 



Basic. J ^ . , j Basalt including plagioclase basalt, 



J I leucite basalt, olivine basalt, 



v Holocrystalline. Gabbro. 



The slides of sedimentary and metamorphic rocks are so few in 

 number that it is quite unnecessary to classify them. The above 

 classification, it will be understood, is merely intended as a con- 

 venient grouping of the rocks referred to in this paper. 



Description of the Slices. 

 It may be well to define a few structural terms, after Rosen- 

 busch, as they are in constant use in studying the slices. 



An authigenic mineral is one that came into existence with, 

 or after the rock containing it. 



An allogenic mineral is one that is of more ancient origin 

 than the rock containing it. 



Minerals bounded by crystallographic contours, being allowed 

 free growth during consolidation, are termed idiomorphic. 



Minerals that have been hindered in their crystallographic 

 development, and whose boundaries therefore are determined by 

 the contours of surrounding minerals are said to be allotrio- 

 morphic. 



Sometimes a mineral which at one time possessed crystallo- 

 graphic form, owes its present irregular and fretted outline to the 

 action of the molten magma in which it floated, prior to solidifi- 

 cation. Such a mineral is said to be corroded. 



The structure in which allotriomorphic masses of augite are 

 penetrated by idiomorphic crystals of felspar is known as ophitic. 



Intersertal structure is characterised by the presence of a 

 hypo-crystalline interstitial substance wedged in between the 

 felspars. 



