394 PROF. T. G. JiONNEY ON THE SO-CILLED ' GNEISS ' 



in. . Over this we worked for some distance, without seeing more 

 of the ' Carboniferous gneiss,' either in crags or tumbled blocks, until 

 we were well beyond the limit laid down on the map. Thereupon 

 we made our waj' to a bridge over the Aar and crossed to its left 

 bank. Hereabonts the granitoid and gneissose rocks in which 

 the upper part of the Haslithal is excavated ^ are well exposed, and 

 on our way back to Guttannen we passed, in the crags which here 

 and there rise near the road, firstly true gneiss, next ' Carboniferous 

 gneiss,' then more true gneiss, much crushed, and lastly more 

 ' Carboniferous gneiss,' though of this, in the field, it was difRcult to 

 be sure. 



Before describing the microscopic structure of the specimens 

 selected for examination, it may be well to state the conclusion at 

 which I arrived in the field. It was as follows : Here and there it 

 is difficult to distinguish between the ' Carboniferous gneiss ' and the 

 normal gneiss, where it has been more crushed than usual, but as a 

 rule the two rocks can be easily separated. The former does not 

 resemble closely any true gneiss known to me in Switzerland. 

 Though composed of the same minerals as the normal gneiss of the 

 district, it is darker in colour and more carbonaceous in aspect. 

 Though it is sometimes banded with micaceous layers like a gneiss 

 or mica-schist, these disappear in other parts of a large block, and 

 the texture altogether' is more variable than is usual in a normal 

 gneiss. The structure throughout is suggestive of a grit somewhat 

 modified by pressure and by slight mineral change, rather than of a 

 rock which has become crystalline in situ. The reasons which led 

 me to this conclusion cannot be fully expressed in words. They 

 depend npon a number of minute peculiarities which one can only 

 learn to recognize after long study of crystalline rocks and of clastic 

 rocks derived from these, in diff'erent aspects and under various 

 eircumstances.- 



At this stage also of the investigation I think it well to call 

 attention to two points in the account which has been published 

 in the ' Beitriige.' ^ One is an omission. The stems in the block 

 from Guttannen are fairly well-preserved, inner and outer casts 



^ These rocks, whicb we exa)uined rather more closely next day in walking 

 vip the Haslithal to the Grriuasel Hospice, bear a general resemblance to those 

 found in the Reuss Valley from some distance below Wasen to near Goschenen. 

 The foliation is very probably, as a rule, the result of pressure ; but a rock like n 

 fine-grained granite appeared in one or two places to cut across the foliated struc- 

 ture and so to be of later date. Nearer to the Grimsel the coarse granitoid rock 

 becomes more distinctly porphyritic, and there are more conspicuous zones of 

 crush. We spent little time over the small infold of rock, marked as ' Sericitic 

 gneiss, &c.,' which crosses the Haslithal rather higher up than the above-named 

 bridge, because, as the publislied section indicates and the tumbled blocks showed, 

 it is a tangled mass of a a ariety of rocks, and thus was not likely to be helpful 

 in regard to tlie question whicli we Avere attempting to solve. 



- An experienced worker gets to know one rock from another as a huntsman 

 knows one hound from another in a pack, or a shepherd knows one sheep 

 from another in a flock, by a number of characteristics which it is often im- 

 .possible to express in words. 



^ Vol. xxiv. pt. iv. p. 161. 



