Correspondence — Mr. John W. Judd. 187 



tricts, they do not occur in the immediate vicinity of granite, but 

 elvan courses are always found near them. The per-centage of silica 

 in the two series of rocks is nearly constant ; the hornblende slates 

 contain about 10 per cent, less silica than the crystalline pyroxenic 

 rocks, and there is an excess of iron oxides to nearly the same extent, 

 their composition in other respects being very similar. The Killas 

 is an acidic rock of essentially different chemical composition. 



2. ''On Columnar, Fissile, and Spheroidal Structure." BytheEev. 

 T. G. Bonney, M.A., P.G-.S. • 



Some of the above structures nave comparatively recently been 

 discussed by Mr. Mallet and Professor J. Thomson. Both these 

 authors agree in attributing columnar structure to contraction due to 

 loss of heat while cooling, but differ in their explanation of cross 

 jointing and spheroidal structure. In this paper it is sought to show 

 that the principle proved by Mr. Mallet to be the explanation of 

 the columnar structure is capable of a wider application. After a 

 brief notice of some instances of columnar striicture, the author 

 described cases of a fissile structure seen in certain igneous rocks 

 (especially in the Auvergne phonolites), closely resembling true 

 cleavage, and often mistaken for it; also the tabular jointing of 

 rocks ; a peculiar form of this, where most of the segments are of a 

 flattened convexo-concave form ; spheroidal structure and cup-and- 

 ball structure. He showed by examples that Prof. Thomson's ex- 

 planation of spheroidal structure was inadequate, and gave reasons 

 for considering all these structures to be due to contraction. He 

 also discussed more particularly the cup-and-ball structure, giving 

 reasons for thinking that the spheroidal and the horizontal fissures 

 were often to some extent independent of each other. 



co:E^:E^:E]s:E^ODN^I^sl^^c:E]. 



ON THE ORIGIN OF LAKES. 



Sir, — I feel sure, that when your correspondents of last month 

 are able to peruse my article on this subject in a somewhat calmer 

 frame of mind, they will be prepared to acknowledge that they have 

 done me (no doubt through inadvertence) some injustice. For, so 

 far from attributing to them the opinion that all lakes were formed 

 by glacial erosion, I was most careful to state (see page 7) that they 

 regard the agent in question " as having originated i\ie greater part 

 of the roch-hasins in which lakes lie." Whatever may be their present 

 opinions — and I venture to hope that we may detect a tendency 

 towards a more cautious treatment of the question — Mr. Geikie and 

 Prof. Eamsay will, 1 am quite satisfied, altogether acquit me of any 

 exaggeration of what they have formerly written on the subject. 

 Unfortunately, the greater part of the contents of your budget of 

 letters is devoted to the rebuke of this purely imaginary act of mis- 

 representation on my part. 



I sincerely trust that, after the admonitions of Prof. Hull, and the 

 amusing burlesque of Mr. Geikie, no one will be so foolishly 

 bold as to attempt to prove the negative of the proposition that 



