GEOLOGICAL HISTORY OF AFGHANISTAN. 



73 



basins. A dark greenish conglomerate, very compact and fairly coarse, 

 occurs in large quantity to the west of the col ; I did not see it in situ, 

 but it evidently belongs to the slate series in this locality, and suggests 

 correlation with that below Kalu. 



In the valley of the Kabul river the rocks consist at first of schistose 

 The valley of the Kabul slates, rapidly giving place to gneiss, schist: and 

 river * crystalline limestone ; the last-named rock forms 



a thick bed extending from Dahan-i-Unai along the left side of the 

 valley down to Sar-i-Chashma. Lower down the valley slates re-appear, 

 but white beds seen high up on the mountain sides probably represent 

 the crystalline limestone. 



What relationship there is between the comparatively little altered 

 slates and quartzites of the Hajigak Kotal and the schists and other 

 highly metamorphosed rocks of the Helmand and Kabul rivers, it is 

 impossible to say. The latter may be merely the altered representatives 

 of the former, but there is no evidence of this, and, judging from the 

 great area covered by these metamorphic rocks, which extend from the 

 Helmand to Kabul, it seems quite likely that they may include meta- 

 morphosed representatives not only of the Hajigak beds but also of the 

 older sedimentary systems. The conglomerates of the Kotal-i-TJnai 

 suggest correlation with those of Ak Robat, whilst the gneisses, schists 

 and crystalline limestones may correspond with those of the valley 

 between Ak Robat and Saraiak in Saigkan, and possibly also with those 

 of the Kabul river to the east of Sarobi (p. 43) . 



IV.— GEOLOGICAL HISTORY OF AFGHANISTAN. 



The geological history of Afghanistan, like the political, is one of 

 instability. Four well-marked unconformities have been recognised. 

 The first occurs between the Khingil series of Khurd Kabul hill and 

 the crystalline limestone and schists of Kabul. It is impossible to 

 estimate the lapse of time represented by this break, since the age of the 

 crystalline rocks has not been determined. It is clear, however, that 

 they were folded and metamorphosed before the deposition of the 

 Khingil limestone since the rocks intrusive in them do not extend up 



