part 4] PLIOCENE deposits of coenwall. 359 



Zircon. — Two very distinct varieties of zircon occur, well-rounded doubly- 

 terminated prisms averaging- 0*3 mm. in length, and short, ' stumpy,' but 

 well-defined prisms with irregular terminations. The former type is usually 

 full of inclusions, and is more characteristic of the basal yellow sands ; the 

 latter variety is almost transparent with only occasional fluid (?) cavities, 

 and is more typical of the red sands and clay. The scarcity of this mineral 

 at the base of the series, its common occurrence at the middle horizons, and 

 its gradual diminution in proportion as the upper beds are reached, considered 

 together, are noteworthy features. 



Kyanite. — These crystals are certainly distinctive, and occur as elongated 

 prisms determined by the (100) cleavage and (001) parting- ; traces of a well- 

 marked (010) cleavage are nearly always present ; the grains have suffered 

 much abrasion, and are on the whole extremely well rounded. They strongly 

 resemble analogous grains found in the Lower Greensand. (Fig. 3, p. 371.) 



Anatase. — Absent in the sands and occurring but rarely in the clay as 

 indigo-blue tabular crystals perfectly formed. Probably secondary, derived 

 from the decomposition of ilmenite. 



E.utile. — This mineral is not so common as might be expected, and well- 

 formed crystals are rare. Generally it occurs in well-rounded ' foxy ' red 

 grains, rudely prismatic, measuring 0*25 mm., and void of any pleochroism. 



Topaz. — Well-rounded, clear, and transparent grains of topaz are charac- 

 teristic of the red sands and clay, but less common in the yellow sand. The 

 grains are variable in size, measuring from O'l to 0-2 mm. in diam.eter, but 

 occasionally attaining- 0*5 mm. They are easily distinguishable from the 

 andalusite by their transparency, absence of pleochroism, and by the biaxial 

 positive interference-figures exhibited by many of them. 



Andalusite. — This mineral constitutes the most important, and at the 

 same time the most interesting, feature of the St. Agnes deposits. It occurs 

 both as irregular rounded grains and as well-formed prismatic crystals, and 

 in most cases exhibits characteristic blood-red pleochroism, more intense 

 and striking than in the case of any other known occurrence of the 

 mineral in British sedimentary strata. The ' rounded grain ' type, which is 

 the commoner mode of occurrence, presents marked subconchoidal fractures, 

 and is usually less turbid with inclusions than the crystals ; the latter 

 consist of well-formed rhombic prisms terminated by (Oil) faces or by the 

 basal pinacoid ; grains flattened parallel to (001) are normal to the acute 

 bisectrix, and show biaxial negative interference-figures. The inclusions are 

 mainly quartz, graphite, and magnetite, and in some cases secondary alteration- 

 products are developed, the chief of which has been recognized as kaolin. 

 The pleochroism and usually turbid character of the mineral serve to differen- 

 tiate it easily from the associated topaz, to which it is only subordinate in 

 quantity in the red- sand horizon. 



Cassiterite. — This mineral is by no means common, and no trace of it 

 was found in the red sand. In the basal sands and also in the clay, however, 

 one or two dusky-brown, rounded, tetragonal crystals were observed, measur- 

 ing about 0*2 mm. in length. 



[b) St. Erth. 



As stated previously (p. 353) six definite litbological horizons 

 were recognized in the St. Erth sands, in the vertical section 

 shown in the Cornish Sand Company's pit ; but only the upper 

 vellowish-brown sand was traceable laterally. The followino- table 



