GEOLOGY OF THE HAYSTACK STOCK 227 



(D, E, F, Fig. 3), while the feldspars and quartz predominate 

 in the rocks around the border (A, B, C). Variety B, containing 

 78 per cent, of quartz and feldspars, occurs at approximately the 

 same elevation as F, which contains only 50 per cent, of quartz 

 and feldspar. If differentiation occurred after the magma had reached 

 the place at which the rocks are now exposed it was such that the 

 elements forming the ferromagnesian minerals moved toward the 

 center of the stock rather than downward in the stock. Accordingly 

 the falling of the heavy crystals does not appear to have been an 

 important process after the magma rose to the place at which the 

 rocks are exposed. 



Fractional crystallization. — Certain intrusive masses appear to 

 have differentiated into rocks of various compositions by simple 

 crystallization during cooling. Since the outer portion of an intrusive 

 body is nearer the colder rocks into which it is thrust, this portion of 

 the intrusive is supposed to solidify first, and the minerals which 

 form first in the rocks would form first of all in the outer zone, and 

 having once commenced to crystallize they would continue to grow 

 while the more soluble constituents would be crowded to the center 

 of the stock. This process, according to Mr. H. S. Washington, 

 accounts for the gradational series of rocks which form the igneous 

 complex at Magnet Cave, Arkansas.^ 



As is shown by the order of crystallization of the minerals (p. 223) 

 quartz and orthoclase formed after the ferromagnesian minerals and 

 plagioclase. If differentiation had been brought about by the process 

 of fractional crystallization, the types in which quartz and orthoclase 

 are present in greater abundance should be the types which were 

 last to form, but the reverse is true if the outer portion of the stock 

 solidified first, for the border varieties A, B, C contain a much larger 

 proportion of quartz and orthoclase and a much smaller proportion of 

 the ferromagnesian minerals than the varieties D, E, F, which occur 

 near the center. 



Fractional crystallization with convection currents. — According 

 to Becker,^ gradational series of rocks which constitute certain dikes 

 and laccoliths can have been formed only through fractional crystal- 



1 Bulletin of the Geological Society of America, Vol. XI, p. 390. 



2 American Journal of Science, 4th Ser., Vol. Ill, p. 21. 



