228 WILLIAM H. EMMONS 



lization with convection currents which during crystallization serve 

 to keep the still liquid central portion of the mass of uniform com- 

 position. The least soluble minerals will go out of solution first 

 and these will form around the outer portion of the stock, while the 

 liquid magma inside continues to supply like material which it 

 deposits on the walls as it moves by them. Applied to the Haystack 

 stock this hypothesis is open to the same objections as the one previ- 

 ously discussed for the minerals which are most abundant in the 

 rocks forming the periphyry are those which formed last in all the 

 rock types of the stock. 



Differentiation prior to intrusion. — Intrusive masses made up of 

 a gradational series of rocks not showing a zonal arrangement across 

 the outcrop are of common occurrence and in most cases they seem 

 to have resulted from differences in the composition of the magma 

 before it came to rest. Mt. Johnson, of the Montenegran Hills, 

 which has been described by Dr. F. D. Adams ^ belongs genetically 

 to the same class, although it is composed of a hollow cylinder of 

 laurvikose inclosing a smaller hollow cylinder of andose, which in 

 turn incloses a cylinder of essexose, the three types grading each 

 into the other. Dr. Adams concludes that before the lava reached 

 its present position, a reservoir somewhere below the mountain had 

 differentiated into a liquid mass of which the heavier portion had the 

 composition of essexose, the upper portion laurvikose, and the 

 central portion andose. The upper portion rising first would form the 

 outer cylinder which in turn would be filled or intruded by andose. 

 Subsequently that would be intruded by essexose. Accordingly the 

 arrangement of the rocks from the bottom up in the earlier body would 

 be the same as that from the center out in the later one. Although 

 the zones are not so well defined in the Haystack stock the rocks of the 

 lowest specific gravity occur near the outer portion of the stock, while 

 the heavier varieties, D, E, and F are near the central portion. 



Since no process of magmatic differentiation appears to have 

 operated after the intrusion to form the gradational series of the 

 Haystack stock, it is probable that the differentiation occurred 

 before the magma was intruded and that the basic central portion 

 represents a magma which was erupted after the more acid portion, 



^Journal of Geology, Vol. XI, p. 22. 



