398 EHRENFELD— JOINTING AS A FUNDAMENTAL FACTOR 



islands, volcanoes, reefs, and so on, may be explained Ijv joint con- 

 trol of the structure of the lithosphere. 



Proposition 6. — That jointing being a constant factor in the 

 structure of the lithosphere and producing lines of easier displace- 

 ment than folding, is one of the fundamental factors in the shifting 

 of land and sea level, whether the shifting be regarded as having 

 taken place under the hydrosphere or in the continental masses. 



Proposition 7. — That jointing should be regarded by students 

 of geophysics as an essential dynamic and constant factor in the 

 behavior of the lithosphere and that it has been so practically 

 throughout all geologic time. 



Proposition S. — Base line or base level of erosion is a term which 

 refers to the results of atmospheric degradation and does not ex- 

 press the final results of continental land surface reduction. Since 

 the relative positions of land and sea are known to be subject to 

 change without any essential change in the character of the rock 

 masses sea level can not be used as a term to express the equivalent 

 of the lowest limit of continental land surface reduction or degra- 

 dation. 



From this there may be deduced principles which are proposed 

 as a 



Law of Joints. 



1. The lithosphere is subject by its nature to the development 

 of lines of weakness or fracturing which in turn develop into actual 

 movable segments. 



These segments or joint lines develop in such regularity of ar- 

 rangement that they may be said to occur in "joint-systems" which 

 are shown at the surface as controlling agents in land erosion and 

 land shaping ; and they act beneath the surface inducing tectonic 

 movements which are independent of atmospheric or marine contact. 



2. Degradation of the lithosphere is fundamentally a factor in 

 its own structure and will occur whenever an agent capable of 

 transporting the movable joint blocks is in contact with the litho- 

 sphere. 



This applies to those portions of the land or rock mass below 

 sea level. 



