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bv Professor Bayley is final, or even that it is acceptable to the 

 majority of petrologists. So far as a distinction between gabbro and 

 diabase is made to rest on purely textural grounds, one being granular 

 and the other ophitic. Professor Bayley's conclusions are good, but 

 the effort to relate these textures to the mode of occurrence of the rock, 

 as intrusive or extrusive, is futile, and the suggestion that the term 

 diabase be applied to holocrystalline extrusive lavas having the com- 

 position of gabbro, and the term basalt to those that are hypocrys- 

 talline (partly glassy) is wholly impracticable. Throughout the 

 chapter the terms melaphyre, augite-porphyrite and diabase are used 

 as synonyms. It does not appear from the description in what sense 

 the term augite-porphyrite is to be understood, since the structure and 

 mineral constituents of these basic rocks at South Mountain are said 

 to be markedly uniform {op. cit. p. 72.); the texture is micro-ophitic 

 and the porphyritic structure is inconspicuous, the largest feldspars 

 being 0.8"'" long. Since it is not possible to prove that the original 

 lavas were or were not glassy, they are classed as having been origin- 

 ally crystalline and for this reason are called diabase in the sense 

 suggested by Professor Bayley. The term apo-basalt could not have 

 been used without question. 



Although the rocks have been greatly altered, enough of their 

 original texture has been preserved to render their identification satis- 

 factory. They had the mineral composition and texture found in 

 many recent basalts. In some cases olivine still remains, in others its 

 outline only is left. In some cases lime-soda-feldspar, augite and 

 magnetite still exist. The secondary minerals formed are quartz, 

 epidote, actinolite, chlorite, and leuxocene. Their relative proportions 

 vary in different places. With very complete change in mineral 

 composition there is surprisingly little change in the texture of the 

 rocks. The chemical analyses of the altered rock show-s considerable 

 divergence in some constituents from the composition of normal 

 basalts. Amygdaloidal, brecciated and tuffaceous forms of the rock 

 occur, which clearly indicate the extrusive character of the lavas. The 

 amygdaloidal varieties have been specially liable to metamorphism, 

 resulting in schists, or slates, spotted where the former amygdales 

 have been dragged into flattened disks. 



The bulletin closes with a summary of the facts and conclusions 

 regarding the occurrence and nature of the rocks, and with a brief 

 notice of the occurrences of similar ancient volcanic rocks in North 



