486 J. J. Mams Teall- — The Lizard Gabbros. 



two or three inches across. These extremely coarse varieties are 

 often distributed with considerable irregularity through large masses 

 of the finer-grained rocks ; not seldom, however, they occur as 

 narrow veins and dykes in the serpentine. The dominant gabbro is 

 a rock which would simply be described as coarsely crystalline. 

 Fine-grained vaiueties are not known. The junction with greenstone 

 or epidiorite is always sharp, and there is no evidence in the Lizard 

 district, so far as I know, of a gradual passage from gabbro into 

 dolerite and basalt. The " greenstones " are undoubtedly altered 

 dolerites, some of which were ophitic, but their junctions with the 

 gabbro are, as already stated, always sharp. 



The mutual relations of the individual constituents in the un- 

 altered 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. Each mineral occurs in the form 

 of crystalline grains. 



In addition to variations of structure depending on variations in 

 the sizes of the individual constituents, there are others, of a most 

 important character, dependent on the mode of arrangement of these 

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

 of the Lizard may be divided into two groups : (1) the massive, and 

 (2) the foliated gabbros. These two varieties 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. It is convenient in speaking of the foliated gabbros to 

 recognize two principal types which may be designated by the terms 

 flaser-gabbro and gabbro-schist. In the flaser-gabbro the parallel 

 structure, though distinct, is not accompanied by any marked fissility. 

 The constituents are white saussurite and dark aggregates of 

 diallage and hornblende. These constituents are more or less 

 lenticular in form and the flat surfaces of the lenticles lie parallel to 

 each other, thus producing the foliated structure. The gabbro- 

 schist is a rock of finer grain, with a strongly-marked schistosity. 

 It can be broken into flat slabs like a hornblende schist. Indeed the 

 rock may sometimes be called hornblende-schist without doing any 

 violence to the latter term. Although it is convenient to use the 

 expressions flaser-gabbro and gabbro-schist, it must not be supposed 

 that there is any hard and fast line between the rocks designated by 

 these terms. They shade into each other by the most insensible 

 gradations. One very interesting structural variety which is, in 

 some respects, intermediate between flaser-gabbro and. gabbro-schist, 

 may be termed augen-gabbro. In this variety the streaks which 

 define the schistosity sweep round " eyes " of diallage, which thus 

 remind one strongly of the " eyes " of felspar in augen-gneiss. (See 

 Plate XIII.) 



We have now to consider the distribution of the different 

 structural varieties. The gabbro to the north of Coverack is on the 

 whole massive; nevertheless here and there foliation makes its 



