part 4] PLTOCEXE deposits of cor]N'wall. 363 



The mixed, iin sorted character of these Crousa- Common deposits 

 venders cpiantitative data of little direct value in this case. Mention 

 has already been made of the variation in grade-size shown hj 

 the constituent particles, and the general coarse texture of the 

 sands has been commented upon ; such variations are reflected in 

 the elutriation-curve for these sediments plotted by Prof. P. Gr. H. 

 Boswell, which compares very unfavourably with the curve of 

 the St. Erth facies.^ It would be expected that the coarser the 

 sample is in the aggregate, the less will be the j'ield of heavy 

 residue, and this was found to be the case in the present instance. 

 The samples collected varied considerably in average texture, and 

 the residues fluctuated quantitatively from O'Ul to 0'2 per cent. 

 The only uniform feature was the mineralogical composition of the 

 sediments, which was found to be constant qualitatively through- 

 out. The characteristic minerals, both in frequency of occurrence 

 and in optical properties, are undoubtedly ilmenite, anatase, and 

 andalusite ; the last-mentioned species particularly predominates 

 in some samples, and in one non-magnetic residue, save for three 

 grains of anatase, it constituted the entire crop. The negative 

 characteristics that should be noted are the extreme rarity of 

 staurolite, the entire absence of kyanite and garnet, and the 

 dropping-off of zircon, when these minerals are considered with 

 reference to the other deposits. 



Ilmenite. — The contrast between this mineral and the magnetite tends 

 to be most marked in these deposits ; whereas the latter frequently occurs in 

 well-recognizable octahedra, the former is invariably irregular, fragmental, 

 and ragged. In some cases the partial aJteration to leucoxene can be seen, 

 and secondary outgrowths of tiny ultra-microscopic crystals (probably of 

 rutile or anatase) from the opaque nucleus are by no means uncommon. 



Tourmaline. — This species is again remarkable for the abundance of 

 basal grains, as at St. Agnes and St. Erth. Prismatic crystals are present, 

 but less frequently than at those localities. The prevalent type is usually 

 rounded but subangular, and many grains show a marked fracturing. The 

 average grain measures O'^- mm. in diameter. 



Anatase. — This species is undoubtedly the distinctive feature of these 

 deposits, and in no other sediment knoAvn to me are the crystals so abundant, 

 so beautifully formed and preserved, and so conspicuous as they are here. 

 The common type is tab'ilar parallel to (001), either as individual crystals or 

 as composite groups of tabular forms in parallel growth. The colour is 

 invariably indigo-blue or greyish-blue ; refractive index extremely high ; bi- 

 refringence high, but tending to be masked by the natural colour ; interference- 

 figure (uniaxial negative) remarkably clear. These crystals average about 

 0*25 mm. in longest diameter. Occasionally fractured grains occur which are 

 almost opaque in white light, but easily studied and identified with convergent 

 light. 



Andalusite. — On the whole, grains of this mineral assume much larger 

 dimensions here than in former instances. They are rarely well crystallized, 

 but tend to occur as rudely triangular fragments, fractured, with ' pitted ' 

 surfaces, and often crowded with inclusions. The blood-red pleochroism is 



^ ' British Resources of Sands & Eocks used in Glass-making ' 2nd ed^ 

 (1918) pp. 30, 31. 



Q. J. G. S. No. 312. 2 c 



