ABSTRACTS OF PAPERS 



63 



in isostatic adjustment. I*eneplains and plateaus can be explained without 

 loss of isostatic equilibrium. Periodic diastrophism is opposed to continuous 

 isostasy, but periodic diastrophism is a theory which is based on incomplete 

 field data. Apparent periodicity may indicate only the natural breaks in the 

 evidence of what might be a complete series of diastrophic movements. Such 

 breaks may be caused by the lack of evidence of the stratigraphic data of 

 earth movements, removal of such evidence by erosion or deposition, and by 

 the occurrence of the deformations wathin the ocean basins. 



Eead by title. 



FIS8ILITY OF ,SHALE: SOME FACTORS CONCERNED IN ITS DEVELOPMENT 



BY J. VOLXEY LEWIS 



(Ahstiacf) 



"Shale" and "'shaly"' — vague and much-abused terms — are applied even to 

 massive claystone, mudstone. and siltstone. and, with more apparent reason, to 

 rocks of alternate lamination : but this structure is obviously stratification 

 and better designated interlaminate (or interfoliate) shale and sandstone, 

 limestone and claystone. etcetera. Few structures are so little understood as 

 the fissility of homogeneous clays and unsorted muds — their splitting into 

 shelly flakes and wedgelike to splintery fragments approximately parallel to 

 the bedding. It is proposed to restrict the terms shale and shaly to such rocks. 



Theoretically, fissility may develop from orientation of the particles during 

 deposition : but with abundant colloids and fine clay and the great water con- 

 tent of the fresh deposits probably this rarely happens. It is. perhaps, more 

 probable in the fresh-water deposits, with their smaller proportion of colloids, 

 than in those of marine origin. Typical shaly structure is. perhaps, generally 

 secondary and attributable chiefly to two gravitational processes: (1) Com- 

 pression (condensation), which eliminates much water, reduces the thickness 

 of strata (even to one-fourth of the original dimensions), flattens particles, 

 and orients flaky and elongate fragments parallel to the bedding. (2) Plastic 

 flow, which turns the longest axes into the direction of movement — down the 

 depositional slope. 



Each process is aided by the other and also by tidal oscillations and by 

 vibrations due to waves, earthquakes, and offshore slumping. Flow is doubt- 

 less largely dependent on colloids (alumina, silica, kaolin-like substances, ferric 

 hydrate, organic matter) and interstitial water, ceasing as particles attain 

 rigid packing. 



Such fissility is metamorphic. In contrast with slaty cleavage, it is parallel 

 to the bedding, since it results, not from tangential stress, but from gravita- 

 tional adjustments due to loading. Much minutely flaky kaolin, chlorite, and 

 mica in shale probably arises from this mild metamorphism. Perhaps natural 

 gas and petroleum also form under these conditions. Expelled by condensa- 

 tion, they migrate, in the presence of water, up the depositional slope and into 

 coarser facies or the porous and cavernous parts of the old land on which the 

 strata overlap. Vibrations and tidal kneading are also doubtless effective 

 factors in the migration. The hydraulic principle probably operates only in 

 the open spaces of the reservoir rocks. 



