72 JOURNAL OF GEOLOGY— SUPPLEMENT 



characteristic minerals. There is no evidence of that tendency 

 toward fairly uniform relative proportions of mineral constituents 

 such as is exhibited in basalt, diorite, and so forth. 



THE AVERAGE IGNEOUS ROCK COMPARED WITH THE PARENTAL MAGMA 



The fact that the average composition of igneous rocks is not 

 basaltic might, perhaps, be considered to preclude the possibility 

 that basalt is the parental magma of all rocks. 



Several calculations have been made of the average igneous 

 rock. The calculation is essentially the averaging of existing 

 rock analyses and gives only the average analyzed rock. Even if 

 weighted according to the relative exposed abundance of types, it 

 is to be noted that the figures obtained would represent only the 

 average exposed rock. It should not be assumed that this average 

 would represent the composition of the general magma from which 

 all rocks were derived. Such an assumption ignores the evidence 

 of such bodies as the Sudbury sheet, the Duluth laccolith, and many 

 others which tell us that the upper part of an igneous body is much 

 more saHc than the magma from which it is formed, especially if it is 

 a large mass. Consideration of this fact with reference to large 

 saHc bodies of which only the upper portion is visible makes it 

 necessary to beheve that the saUc rock is only a light differentiate 

 of the magma from which it formed . This nearly constant tendency 

 of the exposed rock to differ from the magma from which it formed 

 in a special direction makes the average of the exposed rocks differ 

 from the general magma in the same direction. There is therefore 

 nothing contradictory to known evidence in the statement that, 

 though the average of analyzed rocks is tonahtic-dioritic, and a 

 weighted average would be even more acidic, yet the general magma 

 is more basic, viz., basaltic. 



The same considerations apply to the average rock of any 

 particular area. If this average is considered to represent the 

 original magma, and if the composition of any large body of 

 which only the upper part is visible enters into the aver- 

 age, the assumption is implicit in the result of the calculation 

 that the whole of this body is identical with its upper portion. 

 This assumption is unsafe. The earliest of the effusive rocks 



