4i8 



N. L. BOWEN 



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the complicated banding that the finally solidified rocks show. 

 In order to explain the production of banding it is necessary to 

 assume that the action that produced the basin-like shape of the 

 mass was going on continually during the period of crystallization. 

 When the proportion of crystals was small this action could haye no 

 effect on the distribution of materials, but when the proportion of 



crystals began to pass the 50 

 per cent mark it may have 

 had an important effect. 



We shall consider, for sim- 

 plicity and convenience, the 

 possible results of the bending 

 action as apphed to segments 

 of two adjacent layers. The 

 bending will tend to produce 

 a shortening of the' upper 

 layer relative to the lower 

 one, and a shortening thrust 

 is therefore developed in the 

 upper layer. This upper layer 

 may reasonably be supposed 

 to have certain portions that 

 are relatively weak and others 

 relatively strong that may be 

 illustrated diagrammatically 

 as in Fig. 4. It may be supposed too that all of the yielding to this 

 shortening thrust will take place in the weak portions of the layers, 

 and that the strong portions will act as practically rigid plungers. 

 The yielding of the weak portions will likewise relieve the strong 

 portions from any bending force, and a strong portion can 

 therefore retain its original shape, with the consequent develop- 

 ment of a long, narrow space between it and that part of the 

 lower layer immediately adjacent to it. At first thought it would 

 seem impossible that this space could develop, and if by space is 

 njeant empty space this impression would be entirely justified, 

 for the strong segment, though capable of acting as a rigid plunger, 

 would have no real strength as an arch. It should be noted, however, 



Fig. 4. — Showing the effect of warping on 

 a crystallizing mass: (o) shows portion of a 

 mass when it reaches stage at which warping 

 begins to be effective; (&) shows the effect 

 of warping causing compaction of crystals 

 locally and the formation of lenticular pools 

 of residual liquid. Density of stippling is 

 intended to indicate the relative, though not 

 absolute, degree of compaction of crystals 

 in various parts immediately after the action. 

 Final result is a banded rock showing that 

 contrast in composition that existed between 

 liquid and crystals at the time of the action. 



