NATURE OF OPENINGS. 131 



extent of joints along the strike may be many kilometers. While joints 

 are less extensive than faults, they are far more numerous. Probably their 

 number, as compared with faults, more than compensates for their lack of 

 extent. Joints are therefore of very great importance in the flowage of 

 ground water. On the average they may be of even greater importance 

 than faults. Joints, like faults, may connect separated porous strata, but 

 very frequently the joints do not pass through the relatively plastic 

 separating impervious strata, and therefore in this respect are of less 

 consequence than faults. 



Fissility openings usually have less extent than bedding partings, 

 faults, or joints; and the openings are small. While they doubtless have an 

 important influence in water flowage, they are not of such consequence as 

 bedding partings, faults, or joints. 



(2) Openings in which the dimensions of the cross sections are 

 approximately the same are those of the mechanical deposits, including 

 conglomerates, sandstones, soils, tuffs, etc. 



The openings of mechanical sediments have a strong tendency to a 

 definite form, and are continuous. The forms of these openings have been 

 fully discussed by Slichter. The openings alternately narrow and widen. 

 At the wider parts their sections are roughly polygonal, the polygons 

 having more than three sides, and these curved. At their narrowest places 

 the cross sections of the openings approximate triangles, and where the 

 grains are of equal size the triangles are equilateral. The form of the 

 tubes at their minimum is due to the contact of three grains in a plane, 

 the space between which is nearly triangular. (Fig. 3.) 



Professor Slichter has further shown that there are various possible 

 natural systems of packing of particles. In nature one system of packing- 

 may hold for a certain distance, and then be replaced by another system. 

 Within any system of packing all the openings are connected with one 

 another by straight or curved tubes, triangular at their minimum cross 

 section, and no opening is shut off from any other opening. Slichter has 

 shown that in the various natural systems of packing of the particles there 

 is at least one direction in which the tubes are straight; in other words, 

 there is one direction in which a straight wire may be thrust without coming 



a Slichter, C. S., Theoretical investigation of the motion of ground -water: Nineteenth Ann. Kept. 

 U. S. Geol. Survey, pt. 2, 1899, pp. 305-323. 



