part 4] PLIOCENE deposits of CORIS'WALL. 361 



From this it will be seen that the relative proportion of the 

 magnetic to the non-magnetic residue is less, and the percentage 

 of authigenous matter (chiefly limonite) greater than in the case 

 of the St. Agnes deposits. The principal mineralogical features of 

 the St. Erth sands consist in the abundance of tourmaline (variable 

 in colour, size, and shape), in the marked ' flooding ' of well- 

 rounded zircon-crjstals at the upper horizons, in the rare appearance 

 of garnet (absent both at St. Agnes and St. Keverne), in the 

 presence of staui-olite in variable quantity throughout, and in the 

 occurrence of the same type of kyanite (fig. 2, p. 371) as that 

 remarked at St. Agnes, though generally less common here than 

 at that locality. Quartz, muscovite, magnetite, ilmenite, kyanite, 

 and cassiterite all present characteristics similar to the species 

 occurring at St. Agnes (p. 358), and need not be further mentioned. 

 The following minerals, however, are specially to be noted : — 



Garnet. — This mineral, although very rare, is distributed laterally 

 throughout the upper yellowish-brown sand, and also vertically through the 

 upper faeies of the deposits. It occurs as subangular grains never longer 

 than 0"2 mm., with a refractive index of about 1*76, pink in colour, and 

 traversed by numerous intersecting cracks. Its isotropic character is con- 

 sistent. 



Tourmaline. — This most abundant mineral is present both as large and 

 as small grains and prisms, the prisms averaging about 0'3 mm. and the grains 

 0'25 mm. in length. The colours range from grey, blue, purple, green, brown, 

 to yellow, and in many cases parti-coloured grains are seen. A noteworthy 

 feature of the species is its occurrence as basal sections yielding good uni- 

 axial interference-figures ; such basal grains are extraordinarily common at 

 all horizons, and are indistinguishable from similar grains noticed in the 

 St. Agnes deposits (p. 358). Extreme rounding, subangularity, and irregular 

 outline of grains are all developed, the last-mentioned feature being 

 especially noticeable in the reddish-brown sand near the base of the series. 



Staurolite. — Common in the upper beds, but scarcer at the lower 

 horizons, this mineral is well individualized at St. Erth. It occurs as rounded 

 brownish- yellow grains, exhibiting weak pleochroism, high refractive index 

 and, in some instances, biaxial interference-figures. The average grains 

 measure 0*2 mm. 



E pi dote. — Grains of this minferal, when present, are all very similar. 

 They present sharply angular outlines, clear greenish-yellow coloration in 

 transmitted white light, a refractive index equal to that of the garnets, and 

 weak pleochroism. Between crossed nicols, third-order birefringence colours 

 (chiefly blues and yellows) are exhibited, and many of the grains show typical 

 partial interference-figures for cleavage-flakes parallel to (100). 



Zircon. — Reference has already been made above to the ' flooding ' of this 

 mineral at certain horizons, which is its most characteristic feature. Com- 

 pared with the other minerals associated with it, and also with the similar 

 species occurring at St. Agnes, the St. Erth type is much smaller, and much 

 more rounded ; the average size is 0*1 mm. 



E. utile. — In contrast to the St. Agnes deposits, this mineral occurs very 

 commonly at St. Erth ; the prevalent type of grain is a well-rounded, dusky 

 reddish-brown prism, with faint pleochroism and a very high refractive index. 

 Sharply angular and irregular fragments of clear yellow rutile also occur, 

 especially in the uppermost yellowish-brown sands. 



