226 WILLIAM H. EMMONS 



such bodies as are assumed to have resuked from the differentiation 

 of a single magma. The various rocks must be regarded as essentially 

 of the same age, since they are not separated by sharp contacts, as 

 is the case where an igneous rock is intruded by a later one. 



Gradational series of rocks have been recognized and described by a 

 large number of observers. Those who have attempted to explain 

 the phenomenon are in the main agreed that the differences in com- 

 position have been brought about through differences of temperature 

 and pressure, or by the direct operation of gravity during crystalliza- 

 tion. For some differentiated bodies it is assumed that these pro- 

 cesses operated after the homogeneous magma had risen to the place 

 at which the rocks now occur, while for other bodies the magma 

 appears to have been of heterogeneous composition before it rose to 

 its final resting place. These processes have been reviewed com- 

 prehensively by Professor L. V. Pirsson, in a recent publication by 

 the U. S. Geological Survey,^ and to do so here would largely be 

 repetition. Some of them will be briefly stated however and examined 

 not with regard to their general application but with especial reference 

 to the Haystack stock. 



Falling of crystals. — The simplest theory to account for differences 

 in an igneous body which supposedly was once homogeneous is to 

 assume that, of the minerals crystallizing first, the heavier ones 

 sink, and the lighter ones rise much as the constituent minerals of 

 rocks are separated in heavy solutions in the laboratory. Schweig"* 

 has made the suggestion that heavy crystals formed in the early 

 stages .of rock solidification, fall, and are remelted, thus chang- 

 ing th^ composition of the magma at different depths. For the 

 operation of the process the viscosity of the magma must not be too 

 great to allow minerals to settle by their own weight. Magnetite, 

 the heaviest mineral in the Haystack stock, was also one of the first 

 to solidify, and it is present in all varieties in nearly the same quantity; 

 consequently at the time crystallization began the magma must 

 have been too viscous for magnetite to fall. 



The ferro magnesian minerals, which are much heavier than the 

 feldspars, are most abundant in the central portion of the stock 



1 Bulletin No. 237, p. 183. 



2 Neues Jahrhuch filr min. Beil., Bd. 17, p. 516. ■ ^ 



