Yll. 



in the rock as fossils. The solid condition of most of the 

 shells now found fossil in these rocks is due to the great 

 induration which the filling of mud between the valves has 

 undergone. 



By the" time that the mud-beds had completely hardened 

 they had been raised above the surface of the ocean. This is 

 indicated by the crowded condition of the strata about the 

 enclosed concretionary masses, a condition which points to 

 settling or shrinking of the strata, after the loss of the con- 

 tained water, which could only occur after elevation. The 

 elevation was probably due to those crust-movements which 

 are termed "epeirogenic," and which produce extensive 

 changes of level without involving the formation of moun- 

 tains. The mountain-building or "orogenic" movements, 

 which occurred towards the close of Palaeozoic time, and to 

 which the Appalachian ranges owe their existence, unques- 

 tionably affected this region. The initial inclination or 

 "dip" which the strata had at the time of their deposition 

 was accentuated, and the very slight undulations of the 

 strata, which are observable in several places in Western 

 New York, and of which slight indications occur in the 

 Eighteen Mile Creek region, were probably produced at that 

 time. Other structural features, common in mountainous 

 regions were produced, the most pronounced of which are 

 the folds and faults, which occur in a number of places as 

 noted beyond. A fold, as the name implies, is a bend in the 

 strata. A simple arch is called an anticlinal fold (fig. i.). 

 When it is inverted, i. e. when it bends downward, it is called 

 a synclinal fold (fig. ii. ). When the strata are bent upwards, 

 or downwards, and then continue as before, in other words, 

 when the fold represents only half of an anticline or half of a 

 syncline, it is called a monoclinal fold or flexure (fig. iii. 

 and Plate XVI.). 



A fault in stratified rocks, consists of a displacement of the 

 beds along a plane of fracture, which is called the fault plane. 

 Occasionally the fracture or fault plane is vertical (fig. iv.), 



