178 MINERALOGY OF THE LIZARD DISTRICT. 



as will be seen from a careful study of the following. Fig. 3 plate 

 A, from the brow of gabbro above the Coverack valley, shews 

 augite, felspar and olivine associated, but very fresh, and without 

 any perceptible symptoms of change. It was this slide that Prof. 

 Judd stated corresponded with the Tertiary gabbro of the Western 

 Islands of Scotland. Passing around the brow in a N.E. direction, 

 I obtained the next specimen, from which two views are given (fig. 

 4 plate A, and fig. 5 plate B). Here felspar and olivine are found 

 changing into anthophyllite, an acicular variety of hornblende. 

 Anthophyllite is found pervading many of the rocks throughout the 

 Lizard district, and is probably derived from similar passages as 

 this. Continuing the route in a N.E. direction, I obtained the 

 specimen illustrated in fig. 6 plate B, which is full of interest, 

 inasmuch as where the olivine and felspar are in contact the changes 

 into hornblende and marmolite are advancing, as is seen by the 

 broken line of junction, but where felspar and augite, or olivine 

 and augite, are in contact, there is no change to be discerned, the 

 crystalline surfaces being most distinct. 



At Dean Point I obtained the unique specimen, fig. 7 plate B, 

 where a crystal of olivine is held, as if in a vase, by augite, and both 

 of these minerals are surrounded by felspar. Where the felspar comes 

 in contact with the olivine the latter has the appearance of matter 

 in a state of effervesence ; but where olivine comes in contact with 

 augite, the olivine is shielded and protected from corrosion by the 

 augite, as gold or tin protects the baser metals. This is invariably 

 the case, and hence my opinion that the hornblende of this locality 

 is to a great extent derived from olivine. 



At Godrevy Cove denudation appears to have been assisted, 

 judging from the peculiarities of the positions of the higher and 

 lower rocks, for I find the junction of two sheets of unequal ages, 

 in anticlinal positions 5 the ridge has given way and a lower 

 and softer rock has been reached, which forms the bed of a water 

 shed, and reveals the sheets of gabbro on either side of the valley. 

 This outcrop of altered clay-slate, may be seen at about half tide 

 mark on the beach, and is pierced with a network of small veins 

 and dykes of gabbro 5 this position I consider a favorable one from 

 which to calculate the thickness of the basic rocks of the Lizard, 

 which I estimate not to exceed 600 feet. 



