364 



ME. H. B. MILXER OX TUE 



[vol. Ixxviii, 



shown by many individuals, but several of the grains are bereft of this pro- 

 pertj^ particularly the clearer and more irregular types. The refractive index 

 and birefringence are normal, and a few grains exhibit biaxial negative inter- 

 ference-figures. The average diameter of the grains is 0'5 mm., and fre- 

 quently natural grading factors have produced local concentrates comiDOsed 

 'Of individuals almost without exception conforming to this dimension. In both 

 physical and optical characters, this andalusite differs in no fundamental 

 respects from that found in the more northern locahties. 



The other minerals present call for no special remark, save possibly the 

 brookite, of which two grains were identified in the samples taken from 

 the base of the deposits. These grains exhibited characteristics similar to 

 those possessed by the same mineral noted in the St. Erth Beds. 



YI. COMPAEISOX .\]S'D COEEELATIOX OF THE DEPOSITS. 



Average mineralogical composition. 



Species. ' St. Agnes. 



St. Erth. St. Keverne. 



! 



Magnetite 7 



Ilmenite 5 



Garnet — 



1 



7 7 



\ ^ 



8 8 

 4 1 

 4 4 



— 1 

 1 1 

 7 7 



9 4 



Tourmaline 8 



Staurolite 3 



; Epidote 4 



Chlorite 2 



Biotite 2 



Leucoxene 7 



Zircon 5 



Kyanite 6 



Anatase > 2 



5 — 

 15 



Rutile 2 



Brookite — 



6 4 

 1 2 



1 Topaz 5 



4 5 



Andalusite 7 



Muscovite 5 



Corundum ? 



5 9 



4 : 5 



Cassiterite 1 



4 4 ' 



Xenotime ? 1 



? 1 — 



[Conventional signs as in the previous tables.] 



I£ we compare the average mineralogical composition (heavy 

 residues) of the St. Agnes, St. Erth, and St. Keverne deposits, 

 taking also into accomit the crj^stallographical, phj'sical, and op>tical 

 properties of the component species, it soon becomes abundantly 

 clear that, although we are dealing with isolated outcrops, the 

 deposits all present marked petrographical affinities suggesting a 

 close genetical relationship. Eeviewing the results of the inves- 

 tigation from the purely mineralogical standpoint, and in order to 

 validate subsequent questions of correlation and source of origin of 

 material, we have to enquire into the cumulative evidence furnished 

 by the more important mineral species, especially with regard to 

 their frequency or infrequency of occurrence, their persistence in 

 each locality, and the constancy of their mineralogical properties 

 as an indication of consanguinity of origin. 



