P. G. H. Boswell — Petrology of Suffolk Box-stones. 255 



Calcite occurs as grains recrystallized from shell-fragments, but has 

 mostly been removed by solution. 



Apatite is of rare occurrence, probably having been in part removed 

 by solution. It is present as rounded grains which are almost isotropic, 

 and yield a uniaxial figure, being optically negative. Such grains 

 are caused by the imperfect (0001) cleavage. It also occurs in tiny 

 well-formed prisms ("1 X •025 mm.) often terminated by pyramids. 

 These crystals have a clear, limpid appearance, fairly high refractive 

 index, straight extinction, and very low birefringence. 



Quartz is the most abundant constituent of the sandstone, the 

 larger grains being moderately rounded, and the smaller, angular. 

 The only inclusions observed have been parallel rows of tiny bubbles, 

 which appear to be glassy and isotropic. The average dimensions 

 are given on p. 251. 



Andalusite is abundant in the box-stones and is characteristic of 

 the assemblage. A quantity of it (PL X, Fig. 2) was isolated from 

 the non-magnetic portion of the residue of density > 2'83 and <3"33 

 by hand-picking, and the identification confirmed. The usual size of 

 the grains is *18 to "35 mm. diameter, and most contain abundant 

 dark, opaque, rounded inclusions. The grains are bounded by the 

 good cleavage parallel to (110), and a few show pleochroism in 

 a salmon-pink tint for light vibrating parallel to the X-axis, in the 

 direction of which the grains are often elongated. The grains show 

 straight extinction, low polarization colours, and a curved brush in 

 convergent polarized light. 



Staurolite is a characteristic and dominant mineral. It occurs as 

 irregular grains, averaging "23 mm. in diameter, varying in colour 

 from a pale gold to yellow-brown. The fracture is very irregular, 

 some fraying being seen at the edges, and the pleochroism, in tints of 

 golden-yellow, is very marked. Inclusions are abundant, and appear 

 to be glassy and of varied form. (PI. X, Pig. 4.) 



Topaz is rare, and is found as platy colourless grains not unlike 

 mica, determined by the basal cleavage. The refractive index is, 

 however, higher, the optical sign is positive, and the optic axial angle 

 is greater. Topaz is met with in the non-magnetic portion of the 

 residue of density > 3*3. 



Marcasite is not abundant and may be an authigenic constituent. 

 It occurs in small ragged grains, with a yellowish-white metallic 

 lustre by reflected light, but is partly altered to brown limonite. 



Actinolite. — A number of fibrous, colourless elongated grains, with 

 an extinction angle up to 8°, have been referred to this mineral. 



Hornblende, green and pleochroic, with a low extinction angle, 

 occurs frequently as grains determined by the perfect (110) cleavage. 



Epidote is fairly abundant, occurring in yellow-green grains 

 (averaging *2 mm. diameter) which can be ha'nd-picked by reflected 

 light. It is found in the middle magnetic crop with staurolite and 

 tourmaline. The grains are sometimes tabular on account of the basal 

 cleavage, and somewhat rectangular, the imperfect cleavage parallel 

 to (100) being occasionally seen. Many of the irregular grains are 

 pleochroic in yellow-green tints, and show, in convergent light, the 

 emergence of an optic axis. (PI. X, Pig. 3.) 



