318 A Percy JTosJcins — Glauconite from Antrim. 



being yellowish, greyish, or even almost red. These variations of 

 colour, however, mean at least the commencement of decomposition. 

 The size of the grains is usually about one millimetre in diameter, 

 and although much larger masses are sometimes found these are 

 merely agglomerations of the smaller grains. Under the microscope 

 the grains appear quite homogeneous unless some foreign body is 

 enclosed, or, as sometimes occurs, the commencement of decomposi- 

 tion gives a more or less zonary appearance to them. 



The following details have been given me by Miss S. M. Thompson, 

 who kindly forwarded me the specimen used for analysis : " The 

 glauconite grains occur in a cliff of the so-called 'chloritic sandstone' 

 division of the Upper Greensand. The Woodburn River has cut 

 through the Cretaceous rocks in the Knockagh Mountains, and has 

 exposed this face of highly fossiliferous greensand." The specimens 

 were collected from a calcareous sandstone, below the zone of 

 Inoceramus Crispi. 



The method adopted by Miss Thompson for obtaining the grains 

 was this : The rock was pulverized and washed in a muslin bag 

 until the water ran off tolerably' clear ; the powder was dried and 

 sifted through coarse muslin, and then, with the help of a lens of 

 low magnifying power, the grains of glauconite were picked up by 

 means of a very fine sable-brush moistened in a tiny vessel of 

 distilled water, into which the collected grains were dropped from 

 the brush. The field of the lens was large enough to admit of this 

 little vessel of water and a tray of glossy brown paper (upon which 

 pinches of the powdered rock were shaken) at the same time, so 

 that no time was lost in dropping the glauconite into the water 

 as quickly as it was gathered. Great care was taken to have the 

 grains as uniform as possible in size and colour. The sample 

 received for analysis weighed a little over one gramme. It con- 

 sisted of dark green granules of rather less than one millimetre in 

 diameter, very homogeneous and of a rounded contour, intermixed 

 with a few white particles. The grains were soft and readily 

 powdered. The powder was treated with cold dilute hydrochloric 

 acid,^ to remove any residue of calcium carbonate, was well washed, 

 and dried at 90° C. in an air-bath, and the resulting green powder 

 was taken for analysis. 



0-3576grm. was decomposed with moderately concentrated sulphuric 

 acid, the silica separated in the ordinary way, and the solution used 

 for the determination of total iron, alumina, lime, magnesia, potash, 

 and soda by the usual methods. 0-1975 grm. was ignited for combined 

 water and organic water. 0'2780 grm. was decomposed by heating in 

 a current of CO2 with moderately concentrated sulphuric acid, and 

 triturated with standard permanganate of potash for ferrous oxide. 

 The results obtained are given in Column I. 



' Careful subsequent investigations show that any decomposition which may occur 

 during this preliminary treatment with dilute acid has an inappreciable effect upon 

 the figures of the final analysis. Prolonged treatment with dilute acid tends, how- 

 ever, to remove iron, etc., and to heighten the percentage of silica in the residue. 

 Thus a cold solution containing only 0"5 per cent, hydrochloric acid, acting for 

 '69^ hours, extracted 25 per cent, by weight of the constituents of the gkucohite. 



