256 P. G. H. Bosivell — Petrology of Suffolk Box-stones. 



Muscovite is very plentiful in the heavy residues and is present as 

 large flakes, most being rounded, "5 mm. in diameter and '02 mm. in 

 thickness. On account of the large surface area and consequent slow 

 settlement in heavy liquids, much muscovite remains with the light 

 crop of density < 2*8. 



Sphene(?). — Some irregular coffee-brown grains of high refractive 

 index are doubtfully referred to this mineral. 



Orthoclase is fairly plentiful, but is much less abundant than 

 plagioclase. The crystal fragments are often roughly rectangular and 

 are generally much kaolinized. 



Fig. 3. — Kyanite grains from box-stone, Sutton. [206.] (a) Large bent grains. 

 (b) Some of the smaller grains in Fig. 3<x. 



Microcline is not abundant, but grains have been observed with 

 albite-twinning (symmetrical extinction angle 16°) and refractive 

 index below 1 - 525. 



Kyanite is abundant, but in some samples a little less so than 

 andalusite. Many of the grains are of considerable size, and a few of 

 the largest are .>ent in a remarkable manner. Most of the grains are 

 tabular, lying upon the perfect cleavage faces parallel to (100). The 

 (010) cleavage produces long grains, and the basal parting results in 

 approximate rectangularity and cross-cracks. Pleochroism, in ultra- 

 marine blue, is very rare, but examples have been seen with 

 a beautiful colour. The trace of the optic axial plane lies at about 31° 

 to the edge (010) A (100), and plates lying on ( 100) give an emergence 

 of the acute bisectrix. A. few grains resting upon the (010) face 

 have been observed, and these give straight extinction and no figure in 

 convergent light. The average size of the crystals is '44 mm., but 

 some large crystals are as much as l - 7 mm. in length. The latter 

 are sometimes bent in a curious way, and from their extinction it is 

 concluded that the irregularity is due to crystallization along the bent 

 foliation planes of metamorphic rocks. (Text- figs. 3a, i.) 



