Correspondence — Prof. Bonney — Frof. Cole. 43 



coiaK,Es:po2srx)EiNrcE. 



GRANITE CUTTING CRETACEOUS ROCKS— A CORRECTION. 



Sir, — In my Presidential Address to the Geological Society in 

 1885 (Proc. Geol. Soc. vol. xli. p. 76), I speak of having seen in the 

 Alps "perfectly typical granite cutting Lower Cretaceous strata." 

 The remark was founded on a note made in 1874. I am sorry to 

 say that in these words there are two mistakes : the rock is of 

 Tertiary not of Secondary age : the granite is not intrusive. As to 

 the former matter I was misled by a small map, the only one which 

 I then possessed ; as to the latter I fell into a trap. The rock looked 

 like a dyke of grey, not very coarse, granite cutting through a dark 

 schistose rock. 1 was puzzled at not finding more distinct evidence of 

 contact metamorphism ; but this solitary slab-like mass in its general 

 form so closely resembled a dyke, that I did not at that time suspect 

 its true nature. Shortly after the above statement was published, 

 a correspondent (I think Prof. Velain) intimated to me that he 

 believed I had made a mistake ; my own doubts kept increasing ; 

 and last summer I again visited the spot, which is on the road from 

 Sepey to Ormond Dessus. 



The apparent dyke is one of those large erratics which occur not 

 unfrequently in the Flysch of Switzerland, and others may be found 

 at no great distance. How it was that I missed them on the former 

 occasion, and thus failed to have suspicions awakened, I cannot 

 understand, unless it be that changes have been made in the road. 

 Possibly, as I had then worked but little at rocks, something else 

 may have diverted my attention. Be that as it may, there can be 

 no doubt that I made a mistake, and hope that there are not many 

 such on my geological conscience. T. G. Bonney. 



NOTE ON MR. HUTCHINGS'S PAPER ON SOME LAKE-DISTRICT 



ROCKS. 



Sir, — As far as the evidence of the rock-sections goes, the rock 

 from Thornthwaite Crag, described by Mr. W. M. Hutchings, may 

 well be an altered trachyte (Geol. Mag. 1891, p. 543). But the 

 analysis given would indicate a rock nearer andesite, like so many 

 of the " oligoclase-trachytes " of the Auvergne. Considering how 

 trachytes and andesites are associated in the field, and how the same 

 lava-flow may contain varying proportions of porphyritic crystals 

 in various parts, and may consequently yield alkalies in different 

 proportions on analysis of different specimens, I think we must 

 receive with caution the suggestion of an occult rather than a 

 purely chemical cause for the differences between the crystallized 

 constituents of the two types of rock. The analysis referred to 

 by Mr. Hutchings as given in " Aids in Practical Geology " (p. 226 

 of that book) is that of a Sodalite-Trachyte of Ischia. Now I 

 suspect that, had chlorine not been present, this rock would have 

 developed albite and oligoclase in sufficient quantity to bring it 

 at least to the verge of the andesite series. If we call the sodalite 



