REPORTS AND PROCEEDINGS. 



Geological Society of London. 



December 17, 1919. — Mr. G. W. Lamplugh, F.R.S., President, in 



the chair. 



In the absence of the author, the following paper was read by 

 Mr. R. D. Oldham, F.R.S., V.P.G.S. : " A Rift- Valley in Western 

 Persia." By Professor S. James Shand, D.Sc, F.G.S. 



Asmari Mountain, near the oilfields of Maidan-i-Naftun, in the 

 Bakhtiari country of Western Persia, is an inlier of Oligocene 

 limestone among the beds of the Pars System (Miocene), the latter 

 consisting, in the lower part, of bedded gypsum with intercalated 

 shales and a few thin limestones. The mountain is a whale-back, 

 16 miles long and 3 miles wide at the middle, formed by a simple 

 symmetrical anticline plunging at both ends. The north-western 

 end plunges rather steeply, and shows no abnormal structures ; 

 but at the south-eastern end the fold has collapsed along its length 

 for a distance of 3 miles, letting the gypsum beds down into a 

 trough in the limestone. 



This trough is bounded by two main faults hading north- 

 eastwards and south-eastwards respectively, with an average hade 

 of 20°, and marked by steep escarpments. The northern scarp, 

 which lies practically along the axis of the anticline, is at one point 

 500 feet high ; but the southern one, being low down on the flank 

 of the anticline, is much less conspicuous. Besides these main faults 

 there are at least three other big faults parallel to them, which pro- 

 duce minor scarps within the valley ; the valley-floor thus descends 

 in terraces towards the south-west, besides having a general south- 

 eastward inclination of some 10°. Uphill, towards the crest of the 

 mountain, the downthrow of the limiting faults diminishes gradually 

 to zero, and the valley dies out on the broad top of the anticline. 

 Downhill, towards the plunging nose of the anticline, the trough 

 is closed abruptly by a cross-fault nearly at right angles to the 

 anticlinal axis. The length of the whole trough is 2|- miles, and its 

 width half a mile. 



The northern boundary-fault at its maximum has a downthrow 

 of about 500 feet, the parallel faults range from 150 to 200 feet, 

 and the cross-fault throws about 100 feet. 



The gypsiferous beds which once completely filled the trough have 

 been partl}^ removed by erosion, clearly revealing the fault-walls 

 in the lower part of the valley. These fault-scarps in their lower 

 portions are remarkably fresh, and show the smoothed and fluted 

 surfaces produced by the friction of the sliding faces. ' 



The drainage of the faulted region is curious in several respects. 

 The only perennial stream that traverses the valley cuts sheer 

 across it from side to side. Rising in the gypsum beds on the 

 north-eastern flank of the anticline, the stream turns south- 



