Notices of Memoirs — Prof. Bertrand — Structure of Alps. 225 



"the organic constituent of this substance may be considered to 

 be derived from the decay of the various plants which grow on the 

 moist moorlands above, and which, being carried by the waters 

 into the fissures of the granite beneath, combines with the alumina 

 of the decomposed felspar ; and when it reaches the air deposits 

 itself on the roof and sides of the caverns in the form of layers, 

 varying from a line to two or three inches in thickness." 



The mode of occurrence in the cave near Porthcurnow agrees 

 with the description given by Mr. Johnston : over the cave 

 grow furze and bracken, and in the cave itself is a remarkable 

 moss ; fissures are visible in the roof at the farther end. The back 

 of the cave gradually slopes towards the above-mentioned block. 



The Pigotite is also found on the roof of the cave, but there 

 has a different form, the surface being rippled and the ripple-marks 

 being in some cases most distinct in character ; in rare cases the 

 form is serrated or dentated, and is suggestive of a mimicry of 

 certain fossil remains, reminding one of the teeth of Cerafodus from 

 the Ehaetic beds (see Woodcut, p. 224). The frangibility of the 

 mineral makes it difScult to obtain large specimens of any kind. 



In the following month, from another cave only accessible at 

 certain tides, I obtained a small amount of Pigotite with somewhat 

 different characters ; it was more transpai'ent, and in some cases 

 of a rich yellow colour, in others ruby red ; this kind has the 

 general form of an efilorescence. 



It may be hoped that the rediscovery of this mineral may induce 

 some chemist to make a more complete examination of so extra- 

 ordinary a compound. 



isTOTioiES OIF nvcEnvnoiE-s. 



On the Structure of the French Alps. By Professor Marcel 

 Bertrand. Translated from Comptes Eendus. 



EOUE years' work in the Maurienne and Tarentaise at first led 

 me to adopt the views of our Italian colleagues with regard 

 to the "lustrous schists," and to consider them Palaeozoic in age. 

 This year, however, I have found conclusive sections which compel 

 me to return to the old opinion of Lory, and to assign them to 

 the Trias and even in part to the Lias. The proofs, which I shall 

 give elsewhere, are closely connected with general conclusions on 

 the structure of the region studied, and may even I believe extend 

 to the whole chain. It is these which I wish to enumerate briefly 

 here. 



(1) The French Alps possess fan structure. The Carboniferous 

 belt, stretching from Bourg St. Maurice to Brian^on, is the centre 

 of the fan. Along its edges on either hand there is a sort of narrow 

 frontier zone in which the exact direction of overthrusting is un- 

 certain ; but, once this zone is passed, all the folds on the east 



DECADE IV. VOL. I. NO. V. 15 



