MINERALOGY OF THE LIZARD DISTRICT. . 179 



The Manacle Point is now reached, where the older sheets are 

 in bold relief, and are of equal interest to any portion of the Lizard 

 district. These extend, northward, from Godrevy Cove to Porthou- 

 stock Creek, over J-mile in breadth ; eastward, far out into the sea 

 (the dreaded Manacle rocks) ; westward, from the point inland more 

 than 2-miles : in the latter direction it has a dyke-like appearance, 

 but careful views on the held and with the microscope proves it 

 to be the remnant of a sheet of more ancient production than its 

 neighbour, and yet at an earlier date it no doubt resembled the gabbro 

 that now fills the basin S.W. of it. Its principal rock-forming 

 minerals are, sphene, magnetite, labradorite, and horn-blendes after 

 augite and olivine : some of its slides correspond with those from 

 the kernals of gabbro at Coverack, and probably are of the same 

 age. Some of the felspars are almost glassy clear, and in polarized 

 light show grand striation and colours ; others are much changed 

 and opaque. Fig. 8 plate B, a section from a quarry at the head of 

 Porthoustock creek, is the brightest I have obtained from this sheet : 

 the great majority of its minerals show indication of age. 



The banded rocks lying between Porthoustock and Porthallow 

 consist of matter closely resembling that within the Manacle basin 

 from which it undoubtedly overflowed, and, by more rapid chilling, 

 formed its walls ; this again rests nonconformably on the upturned 

 edges of the lower silurian clay slate.* These banded rocks contain 

 a high percentage of the following minerals :-hornblende, augite and 

 felspar, serpentinous bands after olivine, enstatite, etc., with some 

 portions of their minerals yet unchanged. In some cases the 

 enstatites are found changing into schiller-spar, bastite and talc, the 

 augites into hornblendes, and the labradorite and other felspars 

 into saussurite. &c. At the junction of the banded and non-banded 

 rock at Porthoustock, there is what appears to be a small dyke, 

 composed principally of malacolite and felspar, and similar dykes 

 penetrate the banded rock throughout. 



The average specific gravity of the Porthallow stone is 2*984, 

 and that of the Manacle point, 2-964, there being only a difference 

 of 20-oz. to the cubic foot. The sample of rock I tested from the 

 point was a surface one, and that of Porthallow, from the bottom of 



*See map and section in No. 35 Journal Eoyal Institution of Cornwall. 



