DESCRIPTIVE GEOLOGY : SLATE ZONE. II7 



also noticeable near Shawali. This successive overlapping of the 

 shales above the Trias is particularly evidenced in places where a 

 deposit of martite forming the bottom layer of the Spiti shales, and 

 only an inch or two thick, covers the outcropping faces of the Trias, 

 like a stair-carpet. The holes made by the boring molluscs in the 

 same way may be found on both horizontal and vertical surfaces of 

 the rock below. 



The nature of the unconformity is very similar to that between 

 the Productus shales and the Carboniferous quartzite of the Central 

 Himalaya of Kumaun. All the same appearances in the field are 

 generated. 



We have now gone over the structural details of the Sirban hill 



in as concise a way as the nature of the subject 

 General remarks. . 



allowed. I have not thought it worth while re- 

 cording the varying amounts of dip observable at different places, 

 because in a steep rocky hill of this kind with towering precipices and 

 deeply cut-back ravines, so much of the solid geology is seen that a 

 surface dip record would be a feeble and misleading account of the 

 huge folds whose limbs have different inclinations at different depths. 

 If the horizontal section No. I which traverses the most typical part 

 of the mountain be studied in connection with the map and the two 

 geologically-coloured views of it, there should be little difficulty in 

 grasping its main geotectonic features. A few general remarks may, 

 however, be made here. The prevailing direction of strike is north- 

 east and south-west, a law seldom departed from. The prevailing direc- 

 tion of dip is north-west, normal or inverted. But rarely is there a dip 

 to the south-east. No doubt dips to the south-east were once present, 

 but nearly all such have become inverted in the process of the gra- 

 dual growth and development of a set of folded flexures out of a pre- 

 viously gently undulating condition of the rocks. In some cases the 

 inverted limb continues, and in others a reversed fold-fault represents 

 it partly or wholly. Three most striking fold-faults of this kind have 

 come about in tbe Sirban area. The first and perhaps the largest is a 

 matter of inference and follows the north-west edge of the mountain, 



( "7 ) 



