SLATY CLEAVAGE 793 



gin, and the sandstone there is filled with angular fragments of the 

 limestone, which show no traces of solution, or wear by running 

 water. These limestone fragments are themselves frequently in- 

 jected with tongues of the quartz sand. The nature of the contact 

 between the quartz and the limestone, the trituration of the latter, 

 and the inclusion of individual grains of quartz sand in a triturated 

 mass of limestone, the presence of cavities along the contact filled 

 with recrystallized calcite, the presence of elongated limestone 

 "streamers" in the quartz mass near the contact, all "point to a 

 cataclysmic origin of the fissure . . . and a more or less violent in- 

 jection of the sand. ... It seems also certain that the fissure was 

 formed after the deposition of a considerable mass of sand over 

 the . . . limestone, and that the formation of the fissure and the in- 

 jection of sand from above occurred simultaneously. In no other 

 way can we account for the inclusion of horses of the wall rock, 

 often of considerable size, and the injection of the sand into all the 

 fissures and crevices ; nor can we readily explain on any other hy- 

 pothesis the trituration of the limestone along the borders, which 

 clearly indicates a violent contact between the sand and the already 

 consolidated limestone. The supposition that the fissure is due to a 

 violent disruption of the wall ... is further borne out by the nu- 

 merous minute faults which occur in the Waterlimes in the vicinity 

 of the fissure and elsewhere." (Grabau-5 : ^60-^61.) 



14. Slaty Cleavage. Slaty cleavage (Leith-14) is the property 

 of strata to split or "cleave" along certain planes which as a rule 

 have no relation to the planes of stratification. This property is 

 especially well developed in argillutytes, resulting in the formation 

 of "slates," from which the term slaty cleavage has been derived. 

 Slate is therefore a structural term, and has no lithic significance, 

 except in so far as the structure is best developed in the clastic 

 rocks of lutaceous texture, and generally in part at least of argil- 

 laceous composition. This rock structure has been explained as 

 "due to the arrangement of the mineral particles with their longer 

 diameters or radial cleavage, or both, in a common direction, and 

 that this arrangement is caused, first and most important, by paral- 

 lel development of new minerals ; second, by the flattening and 

 parallel rotation of old and new mineral particles, and, third, 

 and of least importance, by the rotation into approximately paral- 

 lel positions of random original particles." (Van Hise-23 : (5j5.) 

 (See also page 770.) 



The original cause of these changes is the lateral compression of 

 the rocks due to orogenic disturbances. Cleavage may be considered 

 as separation of the laminae, potentially developed in the rock. The 



