JO ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



magmas there inevitably must be local variations in temperature in 

 consequence of the liberation of energy during the crystallization 

 of the feldspars and of the accompanying mineral constituents, 

 especially the ferromagnesian silicates. When the crystallization 

 goes on slowly and smoothly, the magma may be expected to cool 

 gradually over the range between the two curves (fig. 22), and a 

 uniform zonal structure result, the extreme viscosity of the liquid 

 operating to prevent any considerable diffusion, resorption, or other 

 modifying phenomena. Any sudden or irregular release of heat 

 which tends to prevent uniform cooling, if it occurs within the range 

 of possible zonal formation, may be expected to result in some varia- 

 tion in the bands; constant temperature for a considerable interval 

 will tend to produce broad bands of uniform composition through the 

 resorption of more calcic crystals already formed, and sharp demarka- 

 ations. In fact, any considerable disturbance, either mechanical or 

 thermal, would probably result in sharp demarkations between bands. 

 Again, if the temperature change should carry the crystals below this 

 critical region, only homogeneous crystals would form unless a near-by 

 release of heat could raise it again. It is easily conceivable that this 

 latter case might produce a partial reversal of the order of the bands. 



In a word, if feldspar crystals begin to form within the range where 

 a change in concentration can occur, zonal structure will probably 

 result, and every change in the temperature will have its effect upon 

 the arrangement of the zones. Long-continued freedom from ther- 

 mal disturbance will produce broad zones, and rapid variation, either 

 continuous or irregular, will produce narrow or sharply bounded ones. 



Inversely, it would appear that whenever a set of thin sections 

 shows traces of zonal structure, and there are few hand specimens in 

 which this structure can not be detected, solidification of the feldspars 

 has taken place within well-defined limits of temperature. When, as 

 in the granites, water vapor or its components have entered into the 

 composition of the magma, it is probable that this range of tempera- 

 ture is a different one, a point to be determined by further researches, 

 but it is evidently practicable to determine for granites as well as for 

 the nearly anhydrous lavas at what temperature the feldspars have 

 solidified, wherever zonal structure can be found. 



