336 Correspondence — R. H. Rastall. 



microtrema and Dicynodon {Tropidostoma) dunni. As pointed out by 

 Lydekker, these belong to the one species, D. microtrema. 



OOIRJRESIE 3 ozisrnDEiiisro JE. 



ANDALUSITE AND CHIASTOLITE. 



Sir, — I have read with much interest the note on the genesis of 

 chiastolite in the May Number of the Geological Magazine. 1 Since 

 I have had some considerable experience in the examination of rocks 

 containing this mineral, may I be permitted to offer a few remarks 

 on the subject? As is well known, chiastolite slate forms a con- 

 spicuous feature of the metamorphic aureole of the Skiddaw granite, 

 especially in its outermost zones, while andalusite is likewise 

 a common constituent of the more highly altered rocks nearer to the 

 outcrop of the granite. 



The author of the paper above alluded to states that inclusions of 

 carbonaceous matter are essentially absent from andalusite : with 

 this conclusion I cannot agree. My experience shows that the 

 structures commonly described as chiastolite are, at any rate in the 

 Skiddaw rocks, shimmer aggregates or pseudomorphs of colourless 

 micaceous and chloritic minerals having the external form of crystals 

 of andalusite and undoubtedly derived from them. This accounts for 

 the inferior hardness and density of chiastolite alluded to by the 

 author of the paper mentioned. In certain bands of the Skiddaw 

 •Slate Series there are numerous large and well-developed crystals 

 of transparent and perfectly fresh andalusite, possessing all the 

 characteristic optical properties of this mineral, with very conspicuous 

 regularly arranged black inclusions : such crystals often show clearly 

 the rose-pink pleochroism of andalusite, exactly as in the granitic 

 rocks in which this mineral likewise occurs. 



I venture to submit, therefore, that more evidence is required for 

 the establishment of a real mineralogical distinction between these 

 two forms, hitherto regarded as varieties of the same species. 



R. H. Rastall. 



Sedgwick Museum, Cambridge. 



miscellaiteoijs. 



Erratum. — In the obituary notice of the late Mr. F. W. Millett 

 some words have been unfortunately transposed, on p. 288 of our 

 June Number ; the first three lines should read : " Mr. F. W. 

 Millett was chiefly known to geologists for his publications on 

 the Foraminifera of the St. Erth Clays, and as an active worker on 

 the more recent forms." 



Addenda. — Please add the following magnifications to the illus- 

 trations in Mr. P. Cr. H. Boswell's paper on "The Petrology of the 

 Suffolk Box-stones (Crag)" (June Number, pp. 250-9): Plate X, 

 Fig. 1, x 26diam. ; Figs. 2-4, X 20 diam. ; Text-fig. 1, p. 251, and 

 Fig. 2, p. 252, x 20 diam.; Text-fig. 3a, p. 256, x 12 diam., and 

 3b, p. 256, X 25 diam. 



1 Geol. Mag., May, 1915, p. 224. 



