P. G. H. Boswell — Petrology of Suffolk Box-stones. 257 



Plagioclase is fairly plentiful, and from its specific gravity and 

 mean refractive index appears mostly to be oligoclase-andesine, but 

 rarely a few grains occur of refractive index greater than 1-560. 

 These are probably labradorite. 



Glauconite. — A very few grains only of this mineral have been 

 observed in a large number of samples examined, and the rarity of its 

 occurrence is in sharp contrast to its abundance in the Diestian of 

 Belgium and Coralline Crag of East Anglia. It may be, as in the 

 case of the Antwerp Crag and the Red Crag of Suffolk, etc., that 

 decomposition of the glauconite has taken place, resulting in the 

 production of the liruonite and secondary silica which are so abundant 

 in the rock. There is no justification for applying the term 

 * glauconitic sandstone ' to the box-stones, as several authors have 

 done. Nevertheless, the association of phosphatic minerals and 

 glauconite is to be expected on genetic grounds. 1 



Limonite is very abundant, giving the sandstone its brown or 

 yellow colour. It coats quartz grains and impregnates the matrix, 

 probably occurring as a cement. Grains which appear by reflected 

 light to be limonite, but which occur in the middle-magnetic crops 

 (limonite is truly non-magnetic) are no doubt actually magnetite, 

 glauconite, or other iron-bearing minerals which have undergone 

 surface decomposition. 



Opal. — A mineral which often coats other grains (magnetite, etc.) 

 and occurs also in the sand of density below 2 - 59 is seen to be clear 

 or pale brownish by transmitted light, but white and milky by 

 reflected light. Its refractive index is below that of Canada balsam 

 (T532), and it appears to be isotropic, so has been referred to opal. 

 It probably acts in part as a cement. 



It is evident from the foregoing list that the box-stones have 

 a peculiarly rich and characteristic mineral assemblage. On comparing 

 the list with the assemblages of other deposits, particularly, perhaps, 

 the various divisions of the East Anglian Tertiaries, we can say 

 that the Box-stone Bed is characterized by the large size of its detrital 

 grains, and its richness, both as regards variety and occurrence, in 

 heavy minerals. The typical minerals are red garnet, andalusite, 

 staurolite, epidote, muscovite, and kyanite, with, less importantly, 

 green amphiboles, brown tourmaline, etc. Butile and zircon appear 

 to be ubiquitous in deposits of all ages, but are really less abundant 

 than usual (especially zircon) in the box-stones. It is rather 

 surprising not to meet with more biotite, particularly as this mineral 

 occurs, though not abundantly, in the Thanet Beds and London Clay 

 in East Anglia, and is plentiful in the later Crag deposits. Opal, 

 limonite, phosphorite, and glauconite are probably authigenic, and 

 marcasite and some of the leucoxene may be also of subsequent 

 formation. 



The mineral assemblage detailed above indicates very clearly that 

 the source of the box-stones is to be looked for in an area adjacent 

 to a massif of crystalline rocks which have undergone regional 

 metamorphism, and bear quartz with strain-shadows, felspars, and 

 such minerals of metamorphic origin as tourmaline, andalusite, 

 1 J. J. H. Teall, Proc. Geol. Assoc, vol. xvi, p. 383, 1900. 

 DECADE VI.— VOL. II.— NO. VI. 17 



