88 



LITHOLOGICAL GEOLOGY. 



Fig. 86. 



Fig. 87. 



In Fig. 86, the lower sandstone layer (1) has no concretions; another (3) contains 

 spherical concretions; in the upper layer (4), an argillaceous 

 sandstone, the concretions are somewhat flattened and 

 coa'lescent; in the shaly layer (2), they are very much 

 flattened, and in its lower part coalescent. 



A radiated arrangement is common when no distinct 

 concretions are formed, as with quartz crystals in irregular 

 cavities. Sometimes different points become centres of 

 radiation, producing a blending of distinct radiations, as 

 in Fig. 87. 



Very many of the mineral species shoot into stellar and 

 globular radiated crystallizations. Others, Like pyrites, 

 readily collect in balls or nodules around a foreign body 

 as a nucleus, or, if none is at hand, around the first mole-< 

 cule of pyrites that commences the crystallization. This 

 tendency in nature to concentric solidification is so strong 

 that no foreign nucleus is needed. The iron ore of coal- 

 regions is mostly in concretions in certain layers of the 

 Coal-measures. The rounded masses often lie imbedded 

 In the clayey layer, or are so numerous as to coalesce into a solid bed. 



Concretions sometimes take fanciful or imitative shapes; and every geologist has 

 had petrified turtles, toads, human bones and skulls, brought him, which were only 

 examples of the imitative freaks of the concretionary process. The turtles are usually 

 what are mentioned as septaria on page 84. Occasionally concretions take long cylin- 

 drical forms, from consolidation around a hole bored by a worm or mollusk, the hole 

 giving passage to the concreting ingredient ; or they derive their form from some rootlet 

 or stem of a plant, in which case they are often branched ; or they were stalactites de- 

 scending from the roof of a cavity. 



b. The Jointed Structure. ■ — Joints in rocks are planes of fracture 

 or divisional planes cutting directly across the stratification and ex- 

 tending through great depths. The planes of division are often per- 

 fectly even, while they may not be open enough to admit the thinnest 

 paper. These joints may be in one, two, or more directions in the 

 same rock, and they often extend, with nearly uniform courses, through 

 regions that are hundreds of miles in length or breadth. The accom- 

 panying sketch represents the falling cliffs of Cayuga Lake, and the 



Fig. 88. 



fortress-shapes and buttresses arising from the natural joints intersect' 

 ing the rocks. The wear of the waters from time to time tumbles 

 down an old surface, and exposes a new range of structures. 



