l6 NEW YORK STATE MUSEUM 



ROCK STRUCTURES 



The physical features associated with the field occurrence of rocks 

 may be considered under the head of structures. Such features 

 include joints, faults and folds, to name some of the more important. 



Joints. One of the most evident characters, common to all rocks 

 whatever their origin, is due to the divisional planes that intersect 

 the bodies so that they are never continuous solids, but are broken 

 up into small blocks. These divisional planes or joints may be but 

 a few inches apart, or they may occur at intervals of 50 or 100 

 feet. In fact, there is every variation almost in their frequency and 

 in their direction with respect to each other. Very commonly there 

 are three sets of joints which intersect at high angles, producing 

 nearly rectangular prisms ; this form is quite characteristic of the 

 sedimentary and of the coarser-grained igneous rocks; but no 

 absolute rule can be laid down for their occurrence. Their attitude 

 with respect to the surface contours and their spacing are important 

 points to be considered in the location of quarry sites, especially 

 if the stone is to be used in dimension or monumental work. 



Joints are in part primary characteristics, that is, they have been 

 produced in the natural course of consolidation of rocks, and in 

 part arise from stresses externally applied after the rocks were 

 consolidated. The former kind is illustrated by the prismatic or 

 columnar jointing found in exposures of fine-grained igneous rocks 

 such as have cooled in narrow channels or near the surface. Fine 

 examples are to be seen in the Palisades diabase. Such jointing is 

 the result of strains set up in the process of cooling and proceeds 

 always at right angles to the exposed surface. 



In the sedimentary rocks, the bedding is a plane of weakened 

 cohesion among the mineral particles and thus marks a direction 

 of potential jointing which probably may result in actual separation 

 on exposure of the beds to drying. The sedimentary rocks also 

 exhibit joints that intersect the bedding at right angles, and in some 

 cases they may be referred to the same cause, contraction on evap- 

 oration of the contained water. 



It is generally considered, however, that joints are mostly second- 

 ary fractures resulting from externally applied stresses. Com- 

 pression arising from crustal readjustments, or torsional and vibra- 

 tory strains incident thereto, is given the greatest importance in 

 recent contributions to the subject- of jointing. The application of a 

 single stress may be resolved into two components at right angles to 

 each other and forming an angle of 45 ° with the direction of the 



