THE MINERAL COMPOSITION AND GEOLOGICAL 
OCCURRENCE OF CERTAIN IGNEOUS ROCKS IN 
THE YELLOWSTONE NATIONAL PARK. 
BY 
Joseph Paxson Iddings. 
[Read before the Society by permission of the Director of the U. S. 
Geological Survey, January 18, 1890.] 
Introduction. 
That there is a connection between the geological occur¬ 
rence of igneous rocks and their crystalline structure is very 
generally admitted ; when by the geological occurrence is 
meant the mode of occurrence of such rocks as dikes, stocks 
or necks, laccolites, or irregularly shaped masses of variable 
dimensions enclosed within other bodies of rock, or as ex- 
travasated masses which assume the form of lava-flows, 
breccias, pumices, and tuffs. 
But the term should be extended so as to include the geo¬ 
logical history of the eruption. For the nature of this rela¬ 
tion is not definite or fixed, since the crystalline structure of 
different rock-masses not only varies with the size and char¬ 
acter of the geological bodies in which they occur, but more 
especially with certain conditions attending the solidification 
of the magmas, including those connected with the whole 
history of their eruption. 
The extent of these variations in structure is becoming 
better understood as the geological investigation of igneous 
23—Bull. Phil. Soc., Wash., Vol. 11. (191) 
