QUARRY MATERIALS OF NEW YORK 19 



For present purposes, it is not necessary to enter upon a dis- 

 cussion of the various types of faults and their effects upon rock 

 structure. They are generally to be avoided in the laying out of 

 quarries. If the aim is to produce crushed stone, their presence 

 may not be objectionable, but even helpful; though care must be 

 used lest the rock be decomposed or so shattered by the faulting 

 as to lose its qualities of hardness and toughness. In mining 

 engineering they are of great importance, and they should be given 

 due consideration also in the plans for permanent foundations and 

 structures, as they mark the lines along which future crustal dis- 

 turbances may occur. 



Folds. The original arrangement of the sedimentary rocks, as 

 determined by their deposition layer by layer upon the flat or 

 slightly sloping sea bottom, is that of a series of parallel and nearly 

 horizontal sheets. Upraisal into land may take place so gradually 

 and uniformly -as to preserve this attitude almost unchanged. Thus 

 the great belts of limestones, shales and sandstones which occupy 

 practically all the State south of the Mohawk and west of the 

 Hudson, show almost no relative disturbance throughout their 

 extent, although they have been elevated through a range of 2000 

 feet or more. When some of the formations are traced eastward 

 from the Hudson toward the New England border, they rapidly 

 lose the appearance of horizontality and assume inclined positions 

 so as to present their upturned eroded edges to the surface. The 

 new arrangement reflects the influence of lateral compression in 

 bending and folding the strata so as to bring them into smaller 

 compass. 



. The development of folds or flexures can be traced in the rocks 

 through all stages from simple to very intricate forms. Every case 

 of folding, however, may be reduced to a variation of two simple 

 basic types, that of the uparched or saddle fold and the inverted 

 type or downfold. The former, called an anticline, is recognized 

 in the field, where the arch itself is concealed or eroded away, by 

 the inclinations of the same beds in opposite directions from the 

 central line or axis. The second type, called the syncline, has 

 inward sloping sides which meet to form a trough. 1 



Simple open folds may have symmetrical limbs which are inclined 

 at the same angles. This is rather exceptional and the sides more 



1 The attitude of folds in the field is found by taking observations of 

 the inclinations and direction of the beds referred to the horizontal plane. 

 The angle of greatest inclination to that plane is the dip; and the direction 

 01 outcrop with reference to the true north is the strike. 



