340 Frof. T. G. Bonney — BeerbacJiite at the Lizard. 



half an inch in length, are " dead white ", like those in the gabbro, 

 and often occur in scattered " coveys ". Occasionally they are 

 joined, and include a small crystal of diallage, so they become difficult 

 to distinguish from the smaller gabbro fragments. In fact, it looks 

 as if the " brown-speckled " rock had occasionally partly melted 

 down bits of the gabbro, destroying the augite rather than the 

 felspar, and had softened and drawn out other patches, so that 

 " a process analogous to what Professor Sollas has described in the 

 Carlingford granites has happened here ". Dykes of a compact 

 '■ greenstone " cut both these rocks, but are neither abundant nor 

 thick ; seldom, I think, more than 4 or 5 feet.^ All along the 

 coast to north of Porthoustock we find (1) gabbro (the dominant 

 rock), (2) next to it the " brown-speckled ", then (3) the greenstone, 

 much less abundant. So there is no justification for colouring all 

 this part greenstone, as in the Survey Map. But I think the beer- 

 bachite at the Lizard must occur in rather thicker masses than it 

 seems to do in Germany. Under the microscope my best slices of 

 beerbachite (I have about seven in all) contain a rather strongly 

 pleochroic hornblende, pale to rich brown, in hypidiomorphic grains, 

 and of rather stumj^y form, together with a granular augite more 

 irregular in outline. The felspar (labradorite), also granular in out- 

 line, is in good preservation, and so is the iron-oxide, which occurs 

 in rather small but irregular, rather numerous grains. I cannot 

 certainly identify hypersthene or segirine. Occasionally the felspar 

 of the beerbachite becomes rather elongated, and the hornblende 

 less granular close to a junction with the gabbro. One slice distinctly 

 shows the partial melting together of the two rocks. Though 

 it does not appear to have any marked chemical difference from 

 several other "greenstones"'^ it seems to be so persistent in its 

 structure and to exhibit such mineral peculiarities, that we must, 

 I think, add beerbachite to the crystalline rocks of the Lizard, as has 

 been already done by Dr. Flett in The Geology of the Lizard and 

 Meneage (1912), where it is described briefly on p. 110 and a figure 

 given at p. 101. 



^ These, as a rule, are not welded to the gabbro, beerbachite, or serpentine, as 

 the troktolite is to the last-named, so I regard the beerbachite as the forerunner 

 of the "greenstone" (basaltic) dykes so common on the eastern, coast of the 

 Lizard. 



^ See the analysis quoted in Rosenbusch's Gesteinslehre and Flett & Hill's 

 Oeology of the Lizard, etc. 



