A. Percy Hosldns — Gtauconite from Antrirri. 317' 



interest because it does not seem unreasonable to believe that the 

 fine striation seen in the felspars of so many contact-rocks is I'eally 

 clue to the same thing, though it is not possible, in most cases, to 

 obtain such good evidence, microscopical and chemical, as this 

 special rock has afforded. 



If we further glance at the analysis we see that it gives a striking" 

 proof of the incorrectness of the supposition, sometimes made, 

 that high percentage of alkalies tends to exclude the formation of 

 sillimanite and andalnsite. Both minerals are abundant in this rock, 

 and yet the alkali-percentage is certainly high. 



If the alkalies are calculated all as orthoclase and albite they 

 correspond to 6 1-72 per cent, of felspars, and we may take this 

 as approximately the amount of those minerals present in the rock.' 



The high percentage of alumina is rather puzzling in a rock which' 

 is an altered ash. Indeed, taking the analj'sis as a whole, had I 

 made it at the time when I looked on the rock as being an altered 

 slate, I should have regarded the chemical composition as strongly 

 confirming this view, as the analysis harmonizes better with this' 

 than with any other supposition. " Indeed, with the exception of 

 the fact that the alkali-total is rather high, and the relative pro- 

 portions of potash and soda are different, the figures are not 

 strikingly unlike those of some of the analyses I gave, in a 

 former paper, of Carboniferous clays. And the resemblance is' 

 still closer to some Lower Carboniferous shales which I have 

 analysed more recently, in which the alkalies range from 7 per 

 cent, to over 8 per cent.^ 



Of course the exact determination of the nature of this bed by 

 Harker and MaiT, from detailed study of the field evidence, is quite 

 conclusive ; and so this rock possesses one more useful point of 

 interest, as it affords an instructive example of the danger and little 

 value which may attach to conclusions drawn from laboratory studies 

 of specimens, without due regard to careful investigation on the 

 ground. 



IX. — On Glauconite from Woodburn, Carrickfergus, 



Co. Antrim. 



By A. Percy Hoskins, F.I.C, F.C.S. 



GLAUCONITE may be described as essentially a hydrous silicate 

 of iron and potash, but of very variable composition, and 

 generally containing varying proportions of other bodies, such as 

 alumina, lime, and magnesia. It is found at different horizons in 

 the whole geological series of I'ocks from the Cambrian up to the 

 most recent Tertiary layers, and, indeed, is particularly interesting 

 in being one of the very few silicates which are in actual process 

 of formation on the sea-bed at the present time. The physical 

 characteristics of glauconite grains are practically the same through- 

 out the series, the normal colour being dark green, but sometimes 



^ See also an analysis by me of one of the altered Silurian "flags" at Shap 

 (Geol. Mag., January and February, 1894), in wbicli the total alkali is close on 

 8 per cent. 



