252 P. G. H. Bostvell — Petrology of Suffolk Box-stones. 



occasionally there is a thin border of subsequently deposited quartz 

 separated from the original grain by a very thin pellicle of brown 

 material. Cracks sometimes occur and are stained brown. Undulose 

 extinction (strain-shadows), characteristic of quartz from crystalline 

 rocks which have undergone dynamic metamorphism, has been 

 observed, but is not common ; one compound grain of quartzite set in 

 the brown matrix was made up of three or four closely locking grains 

 of quartz, one of which showed strain-shadows. 



FlG. 2. — Box-stone, Sutton. [206.] Extreme variety with very little cement. 



In thin sections little felspar is to be seen, the proportion of the 

 grains of felspar to quartz being about one to fifty. Some of the 

 larger grains are almost square, and the mineral is generally full of 

 alteration products ; twinning is usually not seen. 



As a general rule, casts only of the Mollusca are found in box- 

 stones, so that it is interesting to meet occasionally cross-sections of 

 shell-fragments, apparently of bivalves. These sections are long, and 

 somewhat lenticular, having a strong brown-stained border, and an 

 interior made up of an aggregate of small yellow-stained recrystallized 

 calcite grains. 



The deep colour of the matrix seems to mask effectually the heavier 

 minerals which are found on crushing and separating (see pp. 254-7). 

 Certain patches exhibiting aggregate polarization occur here and 

 there, but are too deeply stained to be determinable. Ovoid and circular 

 dark-brown isotropic pellets, probably representing some organic 

 remains, are occasionally seen, but no structure can be made out in 

 them, and the round empty hollows sometimes found may be due to 

 their removal. 



