140 Correspondence — H. W. Bristoiv. 



claim priority for the Blackdown type ; and in case of non-identity, 

 it is the GJ-ault form rather to which the French author's name may 

 be apportioned. However, I have little doubt but that they are one 

 and the same species. The single specimen, imperfect as to the 

 digits, from which J. Sowerby drew ixp his description, seems to me 

 to agree precisely with the Folkestone forms, except that the keels 

 are a little less pronounced; but this is evidently due to the somewhat 

 toned-down state of the specimen ; there are seven to eight threads 

 above the keel, and four between the keels, of which the two 

 central are a little stronger than the remaining two. The surface, 

 instead of being " particularly smooth," as Sowerby says, I should 

 describe as showing traces of oblique cross lines, which have become 

 very obscure through abrasion. I regret that I am unable to com- 

 pare it with the foreign descriptions, but the Museum is quite 

 without the necessary books. 



Bristol Museum, E. B. TawkEY. 



February ISt/i, 1875. 



DEEP BORING IN PRUSSIA. 



Sir, — The experimental boring at Sperenberg having revealed the 

 existence of a deposit of rock-salt, greatly exceeding that of any 

 previously known, I send you some further details, for which I am 

 again indebted to Professor A. von Koenen, of Marburg. 



The boring was begun in gypsum, probably belonging to the Mus- 

 chelkalk. As the boring proceeded, the gypsum was found to become 

 gradually mixed with Anhydrite, and then to pass into pure Anhydrite. 



Still lower, a little rock-salt was met with ; and afterwards at 88*8 

 metres (291-J English feet) pure rock-salt, in which the boring- 

 continued down to 1271-63 metres (4171 English feet) ; no other rocks 

 besides gypsum and salt having been met with. 



Two other borings, at some distance from the first, have reached the 

 rock-salt at 120-6 and 1 1 5 • 8 metres respectively, or at 395^ and 880 

 English feet. 



Prof, von Koenen recommends English geologists, who take an 

 interest in the subject of the increase of the Earth's temperature in 

 proportion to depth, to consult the papers of Obergrath Dunker in that 

 volume of the "Zeitsehrift fur das Berg- Hutten- und Salinen-Wisen 

 in dem Preussischen Staate," which contains an account of the boring, 

 viz. vol. xx. (1872). 



The reduction of Prussian into English feet being incorrect in my 

 former letter, I avail myself of this opportunity of rectifying the 

 mistake: 85, 100, 363A-, 956, 3095, and 4051-6 Prussian feet are 

 equal to 87£, 103, 374, 983f, 8184f, and 4172| English feet 

 respectively. 



The average cost of sinking, therefore, amounted to about £2. Is. 9d. 

 per foot English. 



28, Jermyn Street, H. ~W. BeistoW. 



February, 1875. 



