THE GEOLOGICAL AGE OF CENTEAL AND WEST CORNWALL. 199 



investigation in the field, aided by the examination of thin 

 microsocopic sections and supplemented by numerous chemical 

 analyses. A summary of those observations is given below : — 



Phjsical description. The rocks vary in colour from golden- 

 yellow to greyish or chocolate-brown, with — often — a purplish- 

 tinge in the hardest parts when freshly-broken. Near the suface 

 they are often much decomposed — as at Eoscreage, Beacon Hill, 

 Flushing, Fiddler's Green, Boscolla, and the Gannel. Sometimes 

 this decomposition extends to very considerable depths. AVhen 

 the rock is undecomposed, its appearance is unmistakeable, 

 whether fine or coarse-grained ; where the decomposition is but 

 slight, it often simulates to a cursory view a brown sandstone, 

 for which, locally, it is often mistaken. Where a rock originally 

 fine-grained has become extensively decomposed — as for instance 

 at Flushing, it appears like a brown or yellowish grey tuff full 

 of minute cavities due to the complete solution and removal of 

 some of its crystalline components. 



A tendency to sj)heroidal structure — developed by decom- 

 position — may generally be observed, and this is especially 

 well-marked in the q^uarry section above alluded to, No. 8a, and 

 also in the sections near Trelissick and Channel Creek, as well 

 as in the cliffs below Mawnan Church — places where a con- 

 siderable surface has been for a long time exposed to the action 

 of the tide and spray. We have seen some spheroids at this 

 latter locality not less than eight feet in diameter. 



Microscopic appearance. The true character of the rock is 

 always evident, a. whenever it becomes more than ordinarily 

 coarse-grained, or, b. when unweathered specimens are obtainable. 

 In such cases it is seen to be essentially a comj)ound of 

 plagioclase felspar and brown mica, with some orthoclase and a 

 little quartz — embedded in a crypto-crystalline felspathic base. 

 Occasionally it contains crystals of hornblende, and, in the 

 joints, films of carbonate of line are generally present. 



In some localities masses of a peculiar greasy-looking quartz, 

 somewhat resembling corundum, are found interspersed through- 

 out the mass ; these are occasionally rudely spherical in form — 

 varying in size from less than a hundredth of an inch to upwards 

 of a foot. They often possess a coating of flakes of mica.* 



* Notwitlistandiug its peculiar appearance tlie analysis of this (juurtz reveals 

 nothing peculiar in composition. 



