14 



HAYDEN : GEOLOGY OF NORTHERN AFGHANISTAN. 



The general characters of the highly mineralised crystalline limestone 

 of the Siah Koh arc so similar to those of the spinel- and corundum- 

 bearing limestone of Mogok, that it is difficult to avoid correlating the 

 one with the other, and the temptation to include the Jagdallak rocks in 

 the Archaean system is almost irresistible. Mr. Griesbach, on the other 

 hand, regarded them as metamorphosed calcareous sediments of Carbo- 

 niferous age. Towards the eastern end of the Siah Koh, he found 

 crystalline limestone associated with a grey limestone containing crinoid 

 stems. I think there can be no doubt that the grey limestone referred 

 to is the same as that found by me to the south of the belt of pegma- 

 tite ; I certainly found no fossils in it, but it containedr ods of white 

 calcite such as one often finds in old metamorphosed and foliated lime- 

 stones. 



Mr. Griesbach's specimen from Kala-i-Sher is apparently perfectly 

 identical with the above rock ; it also contains rods of calcite, which I 

 was inclined to attribute to metamorphiem, but, on polishing the rock, I 

 found that these rods undoubtedly represent fragments of crinoids, 

 one of which is a portion of a stem with a quinquelobate axial canal. 

 Evidently, therefore, we are dealing with something very much younger 

 than Archaean ; and if the association of the grey limestone with the 

 ruby- and garnet-bearing rock is such as Mr. Griesbach supposed, the 

 age of the highly mineralised limestone of Jagdallak must be approxi- 

 mately that of the crinoids. In this connection it will be advisable 

 to quote Mr. Griesbach's remarks in full; he says (13, 70) : — 



" A section through the SifJh Koh from south to north presents what appears to 

 be an unbroken sequence of strata. Near the middle of the range, at Bab-i-Kach, 

 a belt of considerable width (at that spot about six miles wide) is formed by a series 

 of metamorphic strata, chiefly mica and hornblendic schists with talcose phyllites. 

 Some beds of finely crystalline grey gneiss beds occur in this series, but on the whole 

 the character of the zone is more schistose. This series is overlaid by highly altered 

 strata, principally limestone beds, within which the old ruby mines of Jagdallak are 

 situated. 



"The limestone belt is quite conformable with the underlying schists and, with 

 them, has a rolling dip to the north and north-east. 



The limestone belt may be traced across the PariDarra (see above), from thence 

 across the high Jagdallak hills, through which it takes an almost dne easterly course 



