MINERALOGY OF THE LIZARD DISTRICT. 177 



mineral composition to it. Fig. i plate A, and fig. 10 plate C, 

 illustrate the predominating minerals of the rocks to the south of 

 Coverack. Here olivine and enstatite are in association, and both 

 changing into serpentine, the former being darker, owing to its con- 

 taining a higher percentage of iron. 



At Coverack, the Lherzolite sheet makes a northerly dip, and 

 is overlaid by alternate sheets of gabbro and serpentinous matter, 

 with many of the original minerals yet unchanged. These sheets 

 are pierced by small dykes of diorite and gabbro, and the whole 

 again overlaid by the St. Keverne gabbro, which is probably the 

 most recent formation of the region. The lower gabbro sheet near 

 Coverack is much decomposed and disintegrated, and along its 

 strike, water has excavated a valley; of this degraded gabbro I 

 obtained a few kernels, to ascertain its constituent minerals. One 

 of these (figure 2, plate A) I found to consist of felspar so far 

 decomposed as to have lost nearly all power of polarization, and of 

 augite changing into hornblende without attacking the felspar, as is 

 the case with hornblende after olivine. In the same slide, beyond 

 the field of view, are crystals of sphene and magnetite. The 

 hornblende in this slide holds a somewhat analogous position in the 

 felspar to the olivine and augite in plate B fig. J, where the olivine 

 has commenced to change into hornblende. General Mc Mahon, 

 has recently given it as his opinion, that " hornblende is a secondary 

 product after augite."* This expression, I believe, is founded on 

 work done in the more southerly portion of the Lizard, where the 

 rocks are older than the gabbro beneath the village of S. Keverne ; 

 the minerals, therefore, would be much more altered, having passed 

 through stages where augite, at last, has succumbed to the solvents 

 of the locality. In the gabbros, north of Coverack, I find the 

 hornblende to be principally a product after olivine, as will be 

 illustrated in detail. 



The gabbro on which the church and village of St. Keverne 

 stand, is the bed rock of several square miles of land. Here I 

 could not successfully follow the line of section (marked in my 

 former paper), on account of the land being in a forward state of 

 cultivation, and was therefore driven to the cliffs, and quarries 

 around the coast, from whence the best results are easiest obtained* 



*Quart. Jour. Geo. Soc, 1889, page 523. 



