752 PRINCIPLES OF STRATIGRAPHY 



The depth at which this occurs varies with the rock substances, 

 being comparatively moderate for plastic substances like coal and 

 clay, and much greater for refractory rocks like quartzites, etc. 

 (Van Hise-34: d//.) 



While universal within the zone of anamorphism, welding is not 

 unknown in the belt of cementation of the zone of katamorphism. 

 Here especially the lutaceous sediments are afifected, the arenaceous 

 and coarser elastics, especially when the particles are of uniform 

 size, having too few points of contact for welding to occur. Thus 

 quartz sandstone of nearly uniform grain may become slightly co- 

 herent by incomplete welding, with cementation weak or absent, and 

 so constitute a "free-stone," so called on account of the ease with 

 which it is quarried and cut. Many of the British cathedrals and 

 abbeys, and some Continental ones as well, are built of rocks of 

 this type, rocks which from their uniformity of grain and ready 

 response to the gravers' tools made possible the elaborate carvings 

 which adorn these structures. In not a few cases this rock seems 

 to have been formed by the induration of former wind-blown sands. 

 The slight cohesion of the round and uniform-grained Sylvania 

 sandstone (Siluric) of Michigan, Ohio and Canada, and of the 

 scarcely more coherent Saint Peter sandstone of the United States 

 furnishes examples of cases where induration has scarcely been 

 effected, though what there is may probably be referred to welding 

 processes. This is seen in the fact that these sandstones are almost 

 absolutely free from foreign matter, which might act as a cement, 

 while, except in rare cases, secondary silica has not been deposited. 



Cohesion may occur in lutaceous sediments without complete 

 exclusion of water. Thus Becker (7:757) has shown that, "when 

 the films of water between the particles become very thin, they may 

 become an important factor in the coherence of the rocks. The 

 molecular attraction of the water films and the adjacent particles, 

 or their adhesion, and the cohesion of the molecules of the films 

 may be sufficient to give the rocks a certain amount of strength." 

 (Van Hise-34 : 5p<5. ) Thus muds and silts welded in this manner 

 may have a marked coherence. 



The squeezing out of the water, in whole or in part, the rear- 

 rangement of particles, and the partial compression of the particles 

 themselves result in a reduction of volume. Thus a considerable 

 reduction in the thickness of a formation may occur. Fossil shells 

 of other organisms in such a formation may be pressed flat or 

 crushed unless previously altered so as to be resistant. The gener- 

 ally flattened or crushed character of brachiopods and other shells in 

 Palaeozoic shales are good illustrations. If, however, a resistant 



