ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 53 



Chromium and titanium have scarcely occurred except as 

 oxides, and in combination with iron, forming the minerals 

 chromite and Ilmenite. Of Ilmenite, rather considerable quanti- 

 ties occur in several parts of the Lizard district, as at Manaccan 

 and Porthalla. It occurs, too, disseminated mostly in small 

 proportions, through the basic intrusive rocks, not only in the 

 Lizard country, but everywhere throughout the district. It 

 is also present, to a small extent, in many of the slates near the 

 veins. As the very rare minerals anatase, Brookite, and rutile it 

 has been found at Delabole and in the Tamar Valley. Chromite, 

 hitherto, has only been found as a rock-constituent very sparingly 

 disseminated in some of the Lizard serpentine. 



GrROUP 4. Phosphates. 



The element phosphorus is very widely spread throughout 

 Cornwall ; scarcely ever, indeed, is it entirel}'' absent, though 

 the quantities present are always very small, and no deposits of 

 commercial importance have been met with. Commonly, it 

 exists in combination with lime as apatite, which can always be 

 seen, in microscopic crystals at least, in the eruptive rocks ; and 

 very often in those which are distinctly of aqueous origin. 

 Crystals of comparatively large size have been found in very 

 many localities, but usually in secondary veins, as for instance, 

 in the granite at Tremearne, St. Michael's Mount, and Stenna 

 Gwynn ; in the killas at St. Agnes, and in the hornblendic rocks 

 of Wheal Cock Carn at St. Just. Many different forms of 

 phosphate of copper have been found in the shallower workings 

 of almost all the copper lodes ; the phosphate of lead is equally 

 common in the lead lodes, and phosphates of iron have been met 

 with with equal frequency in similar situations, and particularly 

 at Botallack and Wheal Owles in St. Just ; at Wheal Jane in 

 Kea ; in the Great Perran Iron Lode ; and in the tin and copper 

 lodes of the Phoenix Mines near Liskeard. Many of the most 

 beautiful specimens in our museums have thus occurred. 



The absolute quantity of phosphorus present in the rocks and 

 veins of Cornwall is probably far greater than that of the 

 fluorine, and some hundreds of times greater than that of the 

 tin. But owing to the absence of considerable local concentra- 

 tions, it is of absolutely no value to the miner, though it is" 



