418 Geological Society : — 



settled for a certainty. Next, the separation of the minerals in a 

 weighed quantity of the powdered rock is undertaken by means of 

 a special separator : the method being illustrated by the example of 

 a rock containing orthoclase (sp. gr. 2*56), quartz (2* 65), andesine 

 (2-67), biotite (3*1), pyroxene (3*3), and magnetite. The first 

 separation would be with a liquid of sp. gr. 2*885, the mean of that of 

 andesine and biotite ; the next with a liquid of sp. gr. 2*66 ; the next 

 2*605, and so on for the other constituents. The separated minerals 

 are dried and weighed, the loss distributed, and the analysis checked 

 by comparing the specific gravity of the rock in bulk with that 

 calculated from the specific gravity and proportion by weight of its 

 constituents. In making choice of particular mineral-analyses for 

 calculating the chemical composition, there are three guiding 

 principles : the analysis should be that of a mineral obtained from 

 the same kind of rock as the one under investigation ; if possible, 

 from the same locality ; and with the same specific gravity. The 

 process was tested on specimens of kentalleuite supplied by Mr. Teall 

 and of gabbros from Skye by Mr. Harker, and in both cases the 

 results compared closely with those obtained from bulk-analysis of 

 the same rocks. A further test was the comparison of the mineral 

 analysis by Miss Davies of a specimen of Devonshire granite with 

 Phillips's published analysis; also of syenite from the Plauenscher- 

 grund, and of tonalite from Adamello, with published analyses. 



February 5th.— J. J. H. Teall, Esq., M.A., Y.P.R.S., President, 

 in the Chair. 



The following communications were read : — 



1. 'The Matrix of the Suffolk Chalky Boulder-Clay.' By the 

 Eev. Edwin Hill, M.A., F.G.S. 



The author has been examining with the microscope washed 

 residues from Boulder-Clays. He is able to group together the 

 specimens from localities along a belt of country from Lowestoft 

 to Bury St. Edmunds, as containing granules of Secondary clays 

 and limestones. Other specimens contain granules which may be 

 the same kind decomposed, others granules of other kinds ; all 

 these lie outside the belt occupied by the group, though some are 

 very near it. The granules of the group, derived from Secondary 

 rocks, may all have come from the west. 



Certain peculiar round grains, found generally, except in the 

 extreme east and north, are also probably from Secondary rocks, and 

 they too point to a western origin. 



The clays of the group, though some occupy the coast-cliffs, con- 

 tain so little sand, that they cannot be supposed to have been 

 brought from the side of the sea, that is from the east. 



All the residues have been examined for coal-dust. Though this 

 is contained in Glacial clays along the eastern coast of England as far 

 south as the Wash, and probably farther, it is either altogether absent 

 from the group, or present only on its eastern edge. It appears to 

 be absent from the clays which border the group on the north. 



These results combined lead to the conclusion that the materials 

 of the matrix in the Suffolk Chalky Boulder-Clay were not brought 



