177 



In jig. 24 the central part represents altered olivine. Surrounding 

 this is a zone of colourless authophylite, and surrounding this again is a zone 

 of radiating actinolite. The remaining portion of the figure represents 

 comparatively unaltered felspar. 



The original felspar-bearing rocks of the gabbro-family occurring in the 

 Lizard peninsula comprised plagioclase-augite rocks, plagioclase-augite- 

 olivine rocks (olivine-gabbros), and plagioclase-olivine rocks (troctolites). 



The replacement of augite and diallage by the various forms of 

 hornblende, and of felspar by saussurite has given rise to a number of other 

 varieties. Two of the most important of these may be designated hornblende- 

 gabbro or gabbro-diorite and saussurite- gabbro or euphotide. It is worthy 

 of note that the saussurite-gabbros appear for the most part to have been free 

 from olivine. One very common rock at Crousa Down is composed of clear 

 glassy labradorite, often occurring in large crystalline plates, and uralitic or 

 actinolite hornblende. It possesses a marked greenish tinge, due to the 

 hornblende. Associated with this rock is a black olivine- gabbro, in which 

 the diallage has retained its character. 



The olivine-graiiis are frequently zoned with the colourless mineral 

 (anthophyllite) and Avith actinolite. 



We have now to notice the structural characteristics of the Lizard 

 gabbros. In the first place we notice varieties depending on variations 

 in the sixes of the individual constituents. In the coarsest varieties, 

 now mainly composed of saussurite and diallage, the individual constituents 

 often measure as much as two or three inches across. These extremely coarse 

 varieties are distributed with considerable irregularity through the finer 

 grained rocks ; not seldom, however, they occur as veins and dykes in the 

 serpentine, and when thi* is the case there is no difference in the texture of 

 the rock occurring at the margins and in the centre of the veins, a fact which 

 suggests that they should be regarded rather as contemporaneous veins than 

 as normal intrusive rocks. 



The normal gabbro is a rock which may be simply described as coarsely 

 crystalline. Fine grained varieties are not known. The mutual relations of 

 the individual constituents in the unaltered gabbros are those of typical 

 granitic rocks ; in other words no single mineral appears to have possessed 

 any special advantage over any other so far as the conditions favourable to 

 the development of crystalline form are concerned. 



In addition to variations of structure depending on variations in the sizes 

 of the individual constituents, there are others dependent on the mode of 

 arrangement of these constituents in the rock-mass. From this point of view 

 the rocks may be divided into two groups : (1) the massive and (2) the 

 foliated gabbros. These tAvo A'arieties are not, however, separated from each 

 other by any hard and fast line. In the massive gabbros the individual 

 constituents are not arranged in any definite manner ; in the foliated gabbros 

 a parallel structure is more or less pronounced. In speaking of the foliated 

 gabbros it is convenient to recognise t\vo principal types \vhich may be 

 designated by the terms Jlafer-gabbro and gabbro-schiat. In the fiaser-gabbro 

 the parallel structure, though distinct, is not accompanied by any marked 



