Sir E. H. Eoworth — Surface Geology of JST. Europe. 257 



pL xix, fig. 1, whose structure is that shown niacroscopically by 

 many gabbros. It is impossible in this case to separate out a number 

 of individual crystals and measure them, so one must scan the 

 picture until one has an idea of the average apparent size of the 

 crystals, fix upon two or three well-defined ones which seem to 

 express this, and measure them. The crystals I fixed upon when 

 measuring this figure were a plagioclase just above the centre, 

 marked " 10" in the key, and a pyroxene to the low left, marked " 7." 

 These measure 19 and 14 respectively in greatest diameter, so that 

 one might quote 16 or 17 as a rough average value. It must always 

 be understood that with a rock of this structure the number given 

 does not represent an actual measurement of crystals, but is only 

 a measure of the general impression the eye forms of its texture, so 

 that it cannot be very definite. 



The ophitic structure usually involves the quotation of at least 

 ihree numbers, for the ophitic plates, the included granules, and the 

 interstitial matter respectively. 



With regard to the time taken by these measurements, I may say 

 that a hand-specimen takes me about three minutes and a slide about 

 five. The most encouraging feature in the method is that the eye 

 can at a glance distinguish between rocks, say, of texture 1'5 and 2, 

 though by the method now in use one could only describe them both 

 as medium-grained. I think it very likely that a little practice will 

 enable one to give roughly the degree of texture of a rock without 

 any measurement at all, as, for instance, to say, " Textui-e granitic, 

 from 2 to 2-5," almost at a glance, though it would of course usually 

 be desirable to check the statement by measurement. It is almost 

 needless to add that the method is applicable to sedimentary rocks 

 equally with those of igneous origin. 



In conclusion, I may say that several of my friends, among them 

 Professor Bonney and Miss Eaisin, have kindly measured some 

 specimens which I placed before them. The coarse-grained rocks 

 presented some difficulties, which led to their not giving quite 

 concordant figures on the first attempt, but the results were on the 

 whole distinctly encouraging, especially among the finer-grained 

 rocks. I venture, therefore, to bring this idea before the notice of 

 petrologists, because, although it does not entirely do away with the 

 personal element in the description of the texture of rocks, it has at 

 least the merit of greatly reducing it, and of so rendering comparable 

 the descriptions of different authors. 



IV. — The Surface Geology of the North of Europe, as illus- 

 trated BY THE ASAR OR OSAR OF SCANDINAVIA AND FlNLAND. 

 By Sir Henky H. Howorth, KiC.I.E., M.P., F.E.S., F.G.S. 

 {Concluded from the May Number, page 206.) 



LET us next turn to the postulated subglacial rivers. How can 

 we conceive of tunnels of ice 100 miles long running in one 

 direction, 300 feet high, and only 20 or 30 paces wide? The 

 difficulty about the provenance of the stones, again, is equally great 



DECADE lY. — VOL. T. — ^NO. ^^l. • 17 



