CEETACEOtIS SERIES. 11 



easily abraded nature of these black shales, and the circumstance that 

 they occur, as a rule, close to dislocations, in the least accessible parts of 

 the mountain, or where slippage and breaking down en masse of the 

 nummulitic limestone tend to conceal them, or where a combination 

 of these causes occurs, places much obscurity in the way of ascertaining 

 the definite limits of the formation, the position of which in the series 

 would even be difficult to decide were it not for its organic remains, 

 which leave no doubt whatever as to its age. The Sirban spiti shales 

 are not so rich in fossils as those of Changligully, referred to in a former 

 notice in these records.* We have found, however, PerispMiictes fre- 

 quens, a fragment of a Belemnite, Inoceramus, and Corlula. 



The unconformity of these shales upon the older rocks is most 

 easily to be observed on the long spur south by west from the summit of 

 Sirban, the surfaces of the underlying triassic limestones being eroded^ 

 pierced by the holes of boring molluscs, and successively overlapped 

 by the Spiti shales. 



On the north-east end of the mountain near Dhumtour these 

 shales have not been observed, the cretaceous beds appearing to follow 

 immediately upon the triassic limestone. The thickness of the group 

 may average from 30 to 50 feet, though the wasting down of their 

 black debris would make it appear much thicker. 



5. Cretaceous. — Towards the upper part, the foregoing group 

 becomes gradually more calcareous and sandy, and a few beds of gray 

 calcareous sandstone supervene. Immediately upon these rests a strong 

 bed of similar petrographical character, but much harder, weathering of 

 a rusty colour, and commonly crowded with fossils of cretaceous genera 

 and species. These rusty beds form a zone from 10 to 20 feet in 

 thickness, and have been observed at several points, the orange colour of 

 the band enabling it to be followed by the eye even at considerable 

 distances. 



* Vide Records, Geological Survey of India, Vol. V, pt. 1, p. 15, 1872. 



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