P. 0. H.Boswell — Petrology of Suffolk Box-stones. 251 



to descriptions, etc., refer to the slides and accompanying mineral 

 separations in the collection preserved at the College. 



Under the microscope the rock is seen to be a coarse sandstone 

 made up largely of grains of clear quartz set in a brown matrix, often 

 so dark in colour as to appear isotropic, but occasionally showing 

 aggregate polarization. There is little uniformity in the size of the 

 grains, the smaller helping to fill up the interstices between the 

 larger, but occasionally there are patches of the brown cement, free 

 from grains, as big as the largest of the latter (Fig. 1). The average 



FlG. 1. — Box-stone, Pettistree Hall, Sutton [204], showing large angular quartz- 

 grains and a high proportion of phosphatic matrix. 



diameter of the fragments is about •3 mm., the smallest being about 

 •12 mm., and the largest up to 5 mm., or even 1 mm., diameter. 

 There is very little material above the last size. The larger grains of 

 quartz are somewhat rounded, and those of felspar almost square, but 

 the smaller grains are subangular to angular, the tendency to rounding 

 diminishing with the size. Some of the smaller grains are astonishingly 

 sharp and angular, most being rather triangular, but a few being of 

 long and irregular, quadrilatei'al form. Usually there is an entire 

 absence of any grading in size. A specimen from Sutton [206] 

 differs in containing smaller grains, averaging -16 mm. diameter, set 

 more closely together, with much less matrix (Fig. 2). 



The quartz is usually very clear and colourless, and free from 

 enclosures. In a few cases, however, parallel rows occur of very 

 small inclusions, like those met witli in quartz from granites and 

 gneisses, and rarely mineral inclusions (zircon, etc., but no sillimanite) 

 have been seen. Some grains are coated with limonitic dust, and 



