270 R. H. Rastali — Minerals of Loiver Greensand. 



grains, they were separated as well as possible by washing in a flat dish 

 and mounted in Canada balsam in the usual way for microscopic exami- 

 nation. The grains vary much in colour, showing many shades of 

 green, also pale brown and nearly colourless. Those with the 

 smoothest surface and least appearance of corrosion show the 

 brightest green colours, while the brown and white grains are 

 obviously altered grains originally green ; all intermediate stages can 

 be seen. Some grains are yellowish brown by transmitted and green 

 by reflected light. It was not possible to determine whether the 

 white or the brown grains represent the greater degree of altera- 

 tion. When small, the brown, and more particularly the white, 

 grains show a peculiar mottled depolarization effect. Some of them 

 show a curious resemblance to grains of turbid felspar, and the 

 discrimination is not always easy. However, the felspars generally 

 show a definite extinction, which the others do not. The most 

 interesting feature of these grains, however, is their form. Many 

 of them have peculiar curved, nodular, or botryoidal outlines, and 

 often a distinctly globular centre. A careful examination of a large 

 number under high magnification, up to 100 diameters, goes far to 

 confirm the idea that many grains of glauconite are in fact casts of 

 the chambers of Foraminifera, while the brown grains so common in 

 the Lower Greensand in surface exposures and shallow pits are 

 undoubtedly formed by oxidation of the green grains of glauconite. 

 This process of change so greatly increases the density of the grains, 

 by the elimination of their lighter constituents, that many of them 

 sink in bromoform until the outer weathered skin has been dissolved 

 away by acid. 



The quartz grains in this specimen show some remarkable features; 

 they are specially notable for their angularity and very irregular 

 shape. A few are subangular, while really rounded grains are rare. 

 The average size of the quartz grains is about - 3 mm. One or two 

 grains contain a few small needles of rutile, the so-called sagenitic 

 quartz, but after prolonged search the presence of tourmaline needles 

 could not be established. Felspar is very abundant, in white or 

 pale pinkish grains; most of them are orthoclase, but microcline 

 was also seen, and a few grains of plagioclase, with extinction angles 

 up to 30°. It was estimated that quartz is the most abundant 

 mineral, making up perhaps half of the total, and that felspar and 

 glauconite are in about equal amounts, while other minerals of any 

 considerable size are scarce : a slide mounted from material without 

 previous separation showed only one crystal of rutile and one or two 

 flakes of muscovite. 



The heavy minerals from this locality are of special interest, since 

 they are abundant and in great variety. The most important 

 minerals quantitatively are kyanite, staurolite, rutile, tourmaline, 

 zircon, and iron ores, but, as will be noted in due course, several 

 rarities are found. It is, however, less confusing and better to treat 

 these separately, together with any notable peculiarities of the 

 commoner minerals. 



Kyanite is abundant in large thick tabular and blade-like crystals, 

 most of them being distinctly rounded and rolled (Fig. 7). Nearly all of 



