SIXTH REPORT OF THE DIRECTOR I909 2*] 



These rock cities consist of immense quadratic blocks of con- 

 glomerate (Olean conglomerate of the geological series) which 

 form the capstone of the broken plateau whereon they rest. 

 The Olean rock city lies at 2400 feet above tide and is highest 

 of them all. The procedure of erosion in cutting out the bound- 

 ing valleys of these hills has left only remnants of this sheet of 

 white quartz conglomerate, now broken up into giant parallel- 

 opipeda by the slow action of atmospheric agencies working 

 along original lines of structure. These structural lines are the 

 vertical series of joint faces produced by shrinkage in and 

 lateral strains upon the rock beds. The Olean conglomerate 

 which composes the Olean rock city, is represented here in 

 very nearly its entire thickness in this State and although for 

 a large part a conglomerate of rounded milk-white quartz 

 pebbles, yet the intervening bands of deposits are fine quartz 

 sand. 



There is nothing above this conglomerate except a few feet 

 of sandy shale which are now embraced within the same geo- 

 logical unit. This Olean conglomerate not only forms the cap- 

 stone of these southern hills, but also the capstone of the New 

 York series of geological formations, it being the highest and 

 latest term in the Carbonic rocks of the State, corresponding in 

 part to the Pottsville division of the Pennsylvania coal measure 

 series. 



The easy decomposition of this rock is due to the fact of its 

 singular weakness of cementation. The pebbles and sand grains 

 are held together, not by any calcareous binder, but by a faint 

 and tenuous deposit of silica derived from the solution and 

 redeposition of the quartz pebbles themselves. So feebly are 

 both conglomerate and sandstones bound that an easy blow, 

 often no more than a hand pressure, will separate a frag- 

 ment. It w r ould appear therefore that time has still been want- 

 ing for a firm cementation by redeposition of a secondary silica, 



