254 P. 0. H. Boswell — Petrology of Suffolk Box-stones. 



under the microscope in transmitted and reflected light. Individual 

 mineral species were isolated by hand-picking under a lens (x 20 

 diameters) in which work a background of black paper was found to 

 be advantageous. Besides the well-known opaque minerals, certain 

 others can be recognized by their characteristic appearance in reflected 

 light, and can be picked out with ease. Muscovite is easily separated 

 by rolling the residue down a rough sheet of paper. Kyanite is clear 

 and glassy, and usually more or less rectangular in form. Andalusite 

 is whitish or dirty-white in appearance, the grains being more 

 irregular. Garnet is pink to colourless, and its fracture causes the 

 mineral to have a well-marked sugary look. Staurolite occurs as 

 deep golden grains, sometimes brownish, but not so metallic-looking 

 as r utile, and epidote as pretty lemon -yellow to yellow-green grains, 

 both minerals being present as irregular fragments. Butile has 

 a characteristic reddish-brown metallic appearance, and is often 

 prismatic. Green hornblende is easily recognized and isolated. 

 Tourmaline has a smoky, bottle-green to brown tint, and frequently 

 the larger grains have a hummocky shape. 



The following is a brief account of the more important minerals : — 



Magnetite is fairly abundant, either as angular fragments or in 

 well-rounded grains about *13 mm. in diameter. A few crystals with 

 octahedron faces have been noted. 



Garnet is an abundant and characteristic mineral, being usually 

 pale pink, but occasionally reddish or colourless. In nearly every 

 case the grains (*2 to - 4 mm. diameter) are very irregular and of all 

 shapes, showing sub-conchoidal fracture and re-entrant angles, but 

 a few occur as platy fragments due to the imperfect (110) cleavage. 

 Rounded grains, if they occur, are rare, and all appear to be entirely 

 isotropic. Inclusions, apparently glassy, occur as well - shaped 

 dodecahedral negative crystals. (PI. X, Fig. 1.) 



Zircon is not so abundant as usual in sediments, in which it is 

 ubiquitous. Small crystals less than -12 mm. long are not common, 

 the average size of the abraded prismatic grains being "13 X '03 mm. 

 Good crystal form and angles are rarely seen. 



Rutile is fairly plentiful, rounded prismatic grains of red-brown tint 

 being common. The yellow variety is rarer, and well-formed crystals 

 are scarce. The grains are larger and more numerous than those of 

 its constant associate in sediments, zircon. 



Ilmenite occurs in profusion, constituting a large part of the first 

 electromagnetic crop. The fragments are usually small and rather 

 angular (-13 mm. diameter), but a few are rounded. They are often 

 surrounded by a comparatively clear border of leucoxene, showing 

 aggregate polarization. By reflected light the mineral has a bright 

 black lustre, and is sometimes striated, but grains are often coated, in 

 part or entirely, with the whitish alteration product, leucoxene. 



Tourmaline. — This mineral is usually of the kind varying from 

 a greyish- to yellow-brown. A very pretty reddish-purple form also 

 occurs. Rounded and broken grains, averaging -18 mm. diameter, are 

 common, but well-formed prismatic crystals are smaller and rarer; 

 these sometimes exhibit hemimorphisra, the prisms being terminated 

 by rhombohedra. 



