16 



IIAYDEN : GEOLOGY OF NORTHERN AFGHANISTAN. 



{infra, pp. 25, 48), in which case its age will be approximately 

 Devonian. 



It is certainly difficult to accept the crystalline schists and lime- 

 stones of Eastern Afghanistan as of Devonian age ; yet if we accept 

 Mr. Griesbach's views as to the gradual passage in the Siah Koh from 

 one formation into another and the absence of any break in the sequence 

 except between the crystalline limestone and the overlying ' gneiss/ 

 there is no escape i'rom such a conclusion, and it will be necessary to 

 regard all the rocks of the Siah Koh as post-Cambrian. On the other 

 hand, Mr. Griesbach does not appear to have been himself entirely con- 

 vinced as to the age of this metamorphic complex, for he has referred to 

 it in one place as " formed of old crystalline rocks, amongst which there 

 may possibly be some palaeozoic outliers. " The ' old rocks ' here may 

 mean his so-called ' gneiss/ which, however, is really a foliated biotite- 

 granite similar to, and probably of the same age as, that of the Hima- 

 laya. 



At the western end of the Siah Koh, the grey limestone, which is 

 presumably the same as Mr. Griesbach's crinoid limestone, has not 

 been found in direct association with the ruby-bearing rock, but is 

 separated from it by a belt of pegmatite, which may quite well represent 

 an important physical break ; it is conceivable that intrusion has taken 

 place along a fault or a thrust-plane, and the approximate juxtaposition 

 of the two limestones would not, therefore, necessarily imply any asso- 

 ciation in time. Mr. Griesbach's study of the whole range, however, 

 was much more extensive than any that I had the opportunity of 

 making, and he makes no mention of any break in the succession ; 

 nevertheless the amount of disturbance to which all the rocks in this 

 area have been subjected is so great that the apparently uninterrupted 

 sequence may quite possibly include an overthrust which has escaped 

 detection owing to the uniformity of dip throughout the section at 

 Kala-i-Sher. 



Turning now westwards we find another series of metamorphic rocks 

 in the Kabul valley, in Logar and in parts of the Paghman range. It 



