A. Percy Hoslcins — Qlauconite from Antrim. 319 



Remembering what has been already said upon the variable 

 composition of glauconite, it is not surprising to find that this 

 example shows some deviations from the figures given for otlier 

 glauconites, although in some particulars these differences are note- 

 worthy. To illustrate this, and also to show how other glauconites 

 vary among themselves, two summaries of analyses are appended. 

 Column II shows the mean of anal3'ses by Sipocz of four samples 

 of marine glauconite from the "Deep Sea Deposit" volume of the 

 "Challenger" Report.^ These were all taken at a depth of 410 

 fathoms, and in lat. 34° 13' S. and long. 151° 38' E. ; yet, taken 

 as they were from practically the same spot, the variations are most 

 pronounced, especially in the essential constituents, silica, ferric 

 oxide, potash, and water. Our Woodburn glauconite shows, however, 

 many differences fi'om all these "Challenger" analyses, and we may 

 note among the essentials that the silica is over 13 per cent, lower 

 than the mean of the four, and more than ten per cent, lower than 

 the least of these ; the ferric oxide, though higher than the lowest, 

 is still more than four per cent, less than the mean ; the potash is 

 nearly double the highest figure given by the four, while the water 

 is somewhere about the mean. One great difference observable 

 is in the ferrous oxide, where, against a maximum of l"9o per cent, 

 in the modern marine glauconite, the Woodburn sample shows 

 16-81 per cent. 



Below I have given a resume of twelve analyses of glauconite 

 from rocks which may be I'egarded as fairly similar to ours, and 

 which are published in Dr. C. Hintze's "Handbuch der Mineralogie."^ 

 In their selection I have been aided by Prof. Cole. Here again, 

 though we might fairly expect a closer correspondence with our 

 own than in the case of the modern glauconite, we find in com- 

 paring them with each other, and with our Woodburn mineral, the 

 same discrepancies in nearly every particular, the silica, for instance, 

 varying from 58 per cent, to a little over 43 per cent., which figure 

 is, however, still somewhat higher than ours. It will be noticed, 

 however, that the ferrous oxide, which is so low in the modern 

 glauconite of Table 1, gives us in Table 2 figures much more nearly 

 agreeing with the glauconite of Woodburn. 



The one point which does seem to come out clearly in this 

 comparison is the relative poverty in silica of the Woodburn 

 specimen, though even that is not much below the lowest in value 

 in the mineral analyses quoted. In this connection it is worth 

 noting that in the " Challenger " results the percentage of silica is 

 lowest, and that of potash is highest in those cases in which the 

 green, and especially the dark-green, casts predominate, casts 

 agreeing in appearance with those composing the whole of our 

 Woodburn specimen. Whether this is merely a coincidence, or has 

 a definite significance, is not as yet certain. The fact that where 

 the dark-green casts are most numerous the mineral particles and 



' Report on scientific results of the voyage of H.M.S. "Challenger" ; "Deep 

 Seca Deposits" (1891), pp. 458-460. 



■^ Hintze, "Handbuch der Mineralogie," p. 850. Analyses I, II, IV-XI, 

 XIII, and XIV. 



