282 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



a dark -green heavy chert and cherty breccia, weathermg brown. A 

 small patch of the same limestone is observed on the north side of the 

 Pass in the descent of the valley between iVsavla and the Gardana i Dii 

 Broder. It rests at a steep angle high up on the slope of the range. 



It is not requisite to multiply examples of this kind, which are 

 exceedingly numerous. 



'* TangSy^ or transverse Clefts in the limestone-saddles. — Before 

 quitting the subject of the Nummulitic rocks, I must not omit to 

 mention the numerous and enormous clefts which pass through the 

 elongated limestone-saddles. 



These clefts, or " Tangs " (as they are called in Persian), are the 

 most peculiar feature of the Nummulitic rocks. 



xA.ll the great rivers which fall into the Tigris from the east rise in 

 the interior of the Zagros ; and, as their course is generally from 

 north to south, they cross the ridges of the great chain diagonally. 

 The manner in which this is effected is very remarkable. Instead of 

 flowing in a S.E. direction along the trough which separates two 

 parallel limestone-saddles, and by this means working out their 

 channel in the soft rocks of the gypsiferous series, or of the alluvium, 

 and afterwards rounding the end of the saddle at the point where the 

 extremity of its visible axis dips under the overlying deposits, each of 

 these rivers takes a direction at right angles to its former course, and 

 passes directly through the limestone-range by means of a " Tang," 

 — which appears almost to be formed expressly for the purpose. On 

 reaching the next gypsum-trough, it follows its original S.E. course, 

 and again passes through the next chain in the same manner, until 

 it reaches the plains of Assyria and Siisiana. The Tangs are not 

 situated at the lowest or narrowest portion of the range, but most 

 frequently divide it at its highest point, and expose a perpendicular 

 section of 1000 feet and upwards. The width of Tangs varies con- 

 siderably, sometimes being exceedingly narrow, and at other times a 

 mile or more across. 



It is quite out of the question to suppose that the rivers them- 

 selves have been in the least degree instrumental in cutting these 

 clefts ; for, if so, we should expect to find a lacustrine deposit in each 

 trough between the limestone-saddles. There are no such deposits. 

 Moreover, if the rivers had been pent up in the troughs, they would 

 certainly have forced their passage through the soft gypsiferous 

 rocks, rather than through the massive crystalline barriers. 



That Tangs are due to the tension of the cooling mass at right 

 angles to the axis of the chains in which they occur, and have been 

 formed during the cooling of the crystalline mass is, I think, self- 

 evident, the entering and re-entering angles on either side of a Tang 

 exactly corresponding with each other. The best example of this is 

 shown in the Derbend, or Pass, between the plain of Denever and 

 that of Chambatan, behind and east of the celebrated rock-sculptures 

 of Bisiitun, near Kermanshah. This tang is no less than ten miles 

 in length, and one and a half in breadth, and has three salient, and 

 two re-entering angles on the N.W. side, with corresponding bays 

 and projections on the other. 



