120 



Chromite and Picotitc. These minerals which are of such constant, 

 occurrence in olivine rocks have been recorded by Professor BONNET as 

 occurring in the Lizard serpentines at Lower Pradanack quarry, Goonhilly 

 Downs, Cadgwith and the Balk. In a variety from Coverack (Cornwall) 

 they occur in the form of irregular masses of a deep rich brown colour when 

 viewed in very thin sections. They have been recorded also by Professor 

 BONNEY from the serpentines of Ayrshire and Aberdeeiishire. 



Secondary Mineral*. In addition to the serpentine and tremolite already 

 referred to we find calcite, steatite, chrysotile and chalcedony occurring 

 as veins in the rock. The examination of thin slices of the Lizard serpentines 

 also shows the frequent presence of a colourless mineral with one strongly- 

 marked cleavage. Sections at right angles to this cleavage are lath-shaped 

 in form and give straight extinction. The polarisation tints are low, and 

 this fact appears to prove that it is not talc, the mineral to which one would, 

 at first sight, naturally refer it. In all cases examined by the author this 

 mineral is so small that cleavage flakes, for more precise optical or chemical 

 determination, cannot be obtained. It may be a colourless chlorite like that 

 of the Eauenthal serpentine, or a mineral allied to the antigorite of the 

 augite- serpentines described by DRASCHE and HUSSAK. (I) Native copper has 

 been worked in serpentine, near Mullion, Cornwall. 



Varieties of British Serpentine. 



In describing the more important varieties of British serpentine it will 

 be convenient to refer to them as they are developed in the several localities. 

 The most extensive development of serpentine in the British Islands 

 appears to occur in the Lizard peninsula of Cornwall. The rock is 

 represented on the geological map as covering an area of about twenty square 

 miles. It forms a gently undulating plateau which is bounded on the east 

 and west by precipitous and mostly inaccessible cliffs. Few exposures of 

 rock occur over the general surface. The rocks associated with the serpentine 

 are gabbro and hornblende schist, and some difference of opinion still exists as 

 to the precise relations of the different rock-masses. Speaking of the hornblende 

 schist DE LA BECHE says : (2) " It supports the great mass of the Lizard serpen- 

 tine with an apparent passage of the one into the other in many places. An 

 apparent passage somewhat embarrassing inasmuch as there is reason 

 to conclude from the grauwacke conglomerate of the Nare Point, in the 

 north part of the Lizard district, that hornblende slates were in existence, 

 forming a surface from which large portions were abraded by the action of 

 water, while the serpentine and diallage rocks of the Lizard were not 

 formed ; for rounded pieces of the hornblende slate though rare are found in 

 the conglomerate, while no trace has yet been discovered of serpentine or 



diallage rock Whatever the cause of this apparent 



passage may have been it is very readily seen at Pradanack Cove, 

 at the coast west of Lizard town, and at several places on the east 



(1) See Ante, p. 113. 



(2) Report on the Geology of Devon and Cornwall. Memoirs of the Geological Survey, 

 Vol. I., 1839, p. 30, 



