J. J. Harris Teall — The Lizard Gabbros. 485 



uralitic variety or round a crystal of diallage partially changed to 

 uralite. The hornblende may be either green, brown, or colourless. 

 One of the green varieties is remarkably brilliant and of that tint 

 which led to the introduction of the term smaragdite. The gradual 

 replacement of diallage by hornblende is a most striking feature in 

 the Lizard gabbros. The two minerals bear an inverse relation 

 to each other so far as distribution is concerned. As the one 

 increases, the other diminishes. 



Olivine does not appear to have been abundant or widely distri- 

 buted in the normal gabbros. It occurs sparingly in an unaltered 

 rock from Coverack, and in this case is perfectly colourless in thin 

 sections and traversed by irregular cracks, along which magnetite 

 has been formed. In some of the more altered gabbros it is repre- 

 sented by serpentinous pseudomorphs. 



The minerals above referred to occur in very different proportions 

 in the different rocks, and in different portions of the same rock-mass. 

 We thus have an endless number of mineralogical varieties. The 

 principal variations are due to the gradual replacement of felspar by 

 saussurite and of diallage by hornblende. The two extreme types 

 may therefore be designated felspar-diallage gabbro, and saussurite- 

 hornblende gabbro ; the former representing the original rock and 

 the latter the extreme of mineralogical metamorphism. Between 

 these two extreme types there are innumerable intermediate varieties. 

 As a rule the change in one mineral is accompanied by a change in 

 the other ; but this is not invariably the case. We may thus have 

 felspar-hornblende gabbros and saussurite-diallage gabbros. If we 

 wish to indicate the mineralogical character of any intermediate 

 form, we can do so by using a compound term made up of the names 

 of the substances occurring as constituents. The number of actual 

 varieties present is the number of possible combinations of. the four 

 principal constituents taken two, three, and four together. The 

 introduction of olivine gives rise to other varieties, but, as already 

 stated, I am inclined to think that this mineral was absent from 

 considerable masses of the original rock. 



If we eliminate hornblende and saussurite, both of which are un- 

 doubtedly secondary minerals, then the original felspar-bearing rocks 

 of the gabbro-type occurring in the Lizard Peninsula must have 

 comprised plagioclase-augite rocks, plagioclase-augite-olivine rocks, 

 and plagioclase-olivine rocks. By the gradual disappearance of the 

 felspar these rocks passed through the picrites into the peridotites, 

 which are now represented by the serpentines. Felspar-bearing 

 serpentines occur at the Rill Head near Kynance. Looking at the 

 Lizard district as a whole, we are struck with many points of resem- 

 blance between it and the Tertiary volcanic district of the West of 

 Scotland described by Prof. Judd. 



We have now to consider the structural characteristics of the 

 Lizard gabbros. In the first place, we notice varieties depending 

 on variations in the sizes of the individual constituents. Such 

 variations are characteristic of all masses of gabbro. In the coarsest 

 varieties the individual crystalline grains often measure as much as 



