On the Serpentine Rocks of the Lizard Distnct. 75 



inland they are generally obscured. Some of them are concealed 

 by debris, &c. ; but the majority pi'ove beyond doubt that the ser- 

 pentine is intrusive. Further, almost everywhere large fragments 

 of hornblende schist are caught up and included in the serpentine. 



Besides the serpentine there is a large mass of gabbro at Crousa 

 Down, and many dykes and veins along the east coast almost to 

 the extremity of the serpentine region. At Coverack Cove, near 

 the above mass, are gabbros of two ages, the older much resem- 

 bling a kind of troktolite ; on microscopic examination it proves 

 to be chiefly plagioclase felspar, augitic minerals (including dial- 

 lage), and olivine partially converted into serpentine ; there is a red 

 and a green variety. The newer, a coarser gabbro, appears to be 

 of the same age as the other veins on the coast, and connected 

 with the main mass. Some remarkable changes have taken place 

 in this also. In certain places it exhibits a separation of its mineral 

 constituents, causing it to resemble a foliated rock. This is proved 

 to be due to pressure at right angles to the structure. The minerals 

 also are often changed. The felspar is replaced by a white granular 

 mineral resembling saussurite ; the diallage (which occurs some- 

 times in very large crystals) is often partially, or even wholly, con- 

 verted into rather minute crystalline hornblende. In these spe- 

 cimens there is no olivine to be distinguished. The great mass, how- 

 ever, is rich in olivine ; yet a weathered specimen from it, resembling 

 in aspect the gabbro of the veins, does not show olivine. Hence 

 the author believes that in certain cases the olivine, instead of 

 being converted into serpentine, aids in forming the hornblende. 

 Further, there are dykes and veins over the same area of a dark 

 trap. Some of these are augitic, others hornblendic. The author 

 believes that, at any rate in certain of these, the hornblende is of 

 secondary formation. On the west coast are veins of granite ; those 

 on the east coast, said to be granite, prove, on careful examination, 

 to be altered rock, remarkably like granite veins, but not really such. 



In discussing the origin of the serpentine, the author called 

 attention to a structure commonly seen, which appeared to be a true 

 " fluidal structure." He then described the result of microscopic 

 examination of many specimens of the Lizard and some other ser- 

 pentines. Commencing with slightly altered Lherzolite (from the 

 Ariege), he traced the change through the older gabbro of Coverack 

 to the serpentine rock of that place, which contains a large quantity 

 of unaltered olivine, and so to other serpentines in which the olivine 

 is quite replaced by the mineral serpentine. He described also the 

 mode of the change. The other minerals found in the serpentine 

 rock are enstatite, varieties of augite, and occasionally a fair quantity 

 of picotite, with, of course, oxides of iron. Hence he concluded 

 that, as had been already shown as regards some other serpentines, 

 that of the Lizard was the result of the hydrous alteration of an 

 olivine rock, such as Lherzolite. 



4. " On certain ancient Devitrified Pitchstones and Perlites from 



the Lower Silurian District of Shropshire." By S. AUport, Esq., F.G.S. 



In this paper the author described a series of ancient vitreous and 



