290 A NATURALIST IN HIMALAYA 



shallow, just like the unstable oceans of to-day. But 

 for immense periods of time that sea must have stood 

 almost unchanged while it deposited the nearly uniform 

 beds of slate. Gradually the crust begins to rise ; the 

 Central Asian sea grows more shallow as it recedes 

 northward ; its bed at length appears above the waters 

 and the slates become a land surface. Now we see in 

 the upturned ends of those slates, denuded by the 

 rains and rivers and unconformably overlain by other 

 sediments, a trace of that ancient land. 



For an unknown time it is worn and denuded by 

 the usual agents that carve the earth. But at last the 

 upheaval ceases ; again the land subsides ; the waters 

 of the Central Asian sea roll back over the sinking 

 shores and Hazara again becomes an ocean. The 

 waters at first are shallow and troubled and are 

 filled with pebbles from the subsiding shores. Slowly 

 the sea deepens, more and more of the earth is 

 submerged, the pebbles are replaced by a finer sand, 

 the sand is followed by a chalky deposit and a deep 

 ocean covers the whole surface. Millions of tiny 

 creatures live in its depths and with their dead bodies 

 build the thick beds of limestone. Throughout the 

 Trias, and for a vast period before the Trias, Hazara 

 lay beneath the sea. Volcanic outbursts then shook 

 the crust and lava flowed over the ocean bed. 



The sea again grows shallow and the waters again 

 recede towards the north. Shales of the Jurassic and 

 the Cretaceous are now depositing in the muddy waters ; 

 ammonites, belemnites and other molluscs flourish 

 beneath the surface and fill the silt with their remains. 

 The ocean bed oscillates, now shallow, now deep, 

 at one time accumulating sand, at another building up 



