468 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Juiie 21, 



trough, it follows for a space its original S.E. course, and again 

 passes through the next chain in the same manner, until it reaches 

 the plains of Assyria and Susiana. The Tangs are not situated at the 

 lowest or narrowest portion of the range, but most frequently di\ide 

 it at its highest point, and expose perpendicular sections of 1000 

 feet and upwards. Thewidth of the Tangs varies considerably ; some- 

 times they are exceedingly narrow, and at other times, a mile or more 

 across. One of these Tang^ which forms the pass between the plain 

 of Denever and that of Chambatan, is 10 miles in length and 1^ in 

 breadth ; it has three salient and two re-entering angles on the N.W. 

 side, with corresponding bays and projections on the other. 



" It is quite out of the question," observes the author, " to suppose 

 that the rivers themselves have been m the least degree instrumental 

 in cutting these cross clefts ; for if so, we should expect to find a 

 lacustrine deposit in each trough, between the limestone saddles. 

 But there are no s^ch deposits. Moreover, if the rivers had been 

 pent up in troughs, they would certainly have forced their passage 

 through the soft gypsum rock, rather than through the massive cry- 

 stalline barriers." 



The "Tangs" are, in the author's opinion, due to tension at right 

 angles to the axis of the chains in which they occur, and were pro- 

 bably produced instantaneously by the cooling of the once heated 

 mass ; whilst longitudinal fractures of the limestone, which also 

 occur, following the axial lines, were probably caused by the resist- 

 ance offered by the overlapping of the numerous beds, during their 

 elevation from an horizontal position. 



As there are no fossils common to the uummulitic and the gypsi- 

 ferous series, so there are none common to the nummulitic and the 

 cretaceous rocks next succeeding, although in this case there is some- 

 times a gradual transition in lithological characters. 



III. Secondly Rocks. 



1. Mr. Loftus refers to the Upper Secondary series a group of 

 limestone and marls, with cretaceous fossils, chiefly exhibited by 

 sections in the Bakhtiyari Moimtains. In descending order, these 

 appear as — 



1. Bituminous marls (sometimes wanting.) 



2. Cream-coloured, fissile, fossiliferous limestone, with flints. 



3. Blue shaly marls. 



4. Limestone with Hippurites. 



The bituminous marls afforded at one place a group of Terebra- 

 tulce, closely resembling Terebratula carnea of the Chalk, together 

 with some Plant-remains, and a small bivalve like a Nucula. The 

 cream-coloured limestone, in the plain of Bishiwah, abounds with 

 crushed specimens of — 



Turrilites: resembling T. tuberculaius. Pecten. 



Ammonites ; one was 2^- ft. in diameter. Turrited univalve. 



A. planulatus. Fuci. 

 Belemnites. 



