250 Mr. Carter on the Delta of the Persian Gulf, 



The existence of a compressed structure round the cavities 

 clearly proves that the diamond has been in a soft state; but 

 it may be shown, from various considerations, that this soft- 

 ness was not the softness produced by igneous fusion, and 

 that it is likely to have been the softness of a semi-indurated 

 gum. I have already stated that no such cavities exist in 

 minerals of igneous origin ; a fact which entitles us to separate 

 the diamond from that class of crystals; and it is equally 

 important to observe that its polarizing structure, which I 

 have studied with peculiar care in a great variety of specimens, 

 connects it closely with amber and indurated gum. From 

 such substances, indeed, it differs in having a distinct crystal- 

 line form ; but in the mineral resin called mellite we have an 

 equally distinct crystalline form, though there can be little 

 doubt, both from its composition and its locality, that it de- 

 rives its origin from the vegetable kingdom. 



XXX. On the Ancient and Modern Formation of Deltas in the 

 Persian Gulf by the Euphrates and Tigris, in answer to 

 Mr. Beke. By W. G. Carter, Esq. 



[Continued from p. 202, and concluded.] 



CINCE writing the foregoing remarks, I have seen Mr. 

 ^ Beke's paper on what is termed the geological evidence 

 of the advance of the land at the head of the Persian Gulf, 

 which commences by bringing again into notice the single 

 passage of the historical, in which Pliny, after mentioning three 

 different admeasurements from the gulf to Charax, a port 

 lying near the course by the Euphrates, to Babylon, — the 1st, 

 and shortest, made in the time of Alexander the Great; the 

 2nd, and longer, furnished by Juba; and the 3rd, and longest, 

 being of Pliny's time, — the historian, very remarkably, goes on 

 to account, as it seems, for these varying estimates by saying 

 that the silt of the rivers had made additions to the land*. 

 The peculiarity is, that in the preceding chapter he had given 

 measurements of the whole distance from the gulf past Charax 

 up to Babylon, which presented a totally different result. 

 For the first there, is the longest, and is also made in the timeof 

 Alexander, and the second is shorter, and yet also is furnished 

 by Juba; and the inference there is the very natural one, that 

 the account being thus discordant, he had not been able to 

 determine the distance f. 



* Pliny, Hist. Nat., lib. vi. cap. 27. 



t Errata in former part, last Number: — p. 200, note, for" Bosra, 12 miles 

 below Khorna," read 42; p. 198, note, for " 35 miles" read 37; and p. 199, 

 for have read has. 



