Vol. 52.] AND INTRUSIVE IGNEOUS ROCES. 609 



proper intrusive connexions, it is clear that, since all the portions 

 of magma erupted from a reservoir of magma may not reach 

 the surface of the earth, the series of lavas which have been 

 extruded may be a partial series, imperfect at the beginning or 

 end, or anywhere between. And the same must be true for any 

 disconnected group of intruded bodies. 



On all these points we agree. The points of difference seem 

 to be the following: — That the order of primary differentiation and 

 eruption can be learned only from a study of large bodies of deej - 

 seated igneous rocks : and consequently that the bodies of extrusive 

 rocks from which my conclusions were drawn are not adequate for 

 the solution of the problem ; that the range from intermediate to 

 greater and greater extremes is not of verj r general application, and 

 that the order of succession of the rocks at Electric Peak represents 

 a more general law, namely, that the succession is usually from 

 basic to acid, often followed by basic as the last. 



In order to defend my position from this friendly attack, it is 

 necessary to picture as vividly as I can the relation of the igneous 

 rocks at Electric Peak to all those that took part in the great 

 series of eruptions which occupied almost the whole Tertiary period, 

 and spread themselves over an enormous territory lying in Montana, 

 Wyoming, and Idaho. 



The extent of this great period of volcanic activity, though vast, 

 is well marked in this region. 1 During the longer periods of time 

 during which Palaeozoic strata were being deposited, no outburst of 

 igneous rock took place. The same is true for all the Mesozoic 

 period until after the Laramie coal-bearing layers had been formed : 

 the whole of this great series being nearly conformable throughout. 

 At the end of Cretaceous time, there were great orographic move- 

 ments, by which the region was profoundly faulted and dislocated. 



Volcanic action began at this time, and fragments of igneous 

 rocks are included in the sandstones immediately overlying the 

 coal-bearing Laramie. 2 There was also extensive denudation 

 whereby the sedimentary strata were in places entirely removed from 

 off the underlying crystalline schists. Into these disturbed strata 

 the igneous material was forced, and upon the uneven surface of 

 the country it was spread out, covering crystalline schists in one 

 place, and upturned limestones and sandstones in another. In most 

 cases the character of the eruptions was extremely violent. At 

 first they were largely explosive, shattering the surface-rocks, 

 whether gneiss or limestone, and scattering them broadcast to form 

 the first layer of tuff-breccia, or to be intimately mingled with 

 fragments of lava. The explosive character prevailed until a great 

 accumulation of tuff-breccia formed a chain of lofty volcanoes com- 



1 Hague, A, ' Geological History of the Yellowstone National Park,' Trans: 

 Am. Inst. Mining Engineers, vol. xvi. (1888) p. 783; also Compte-rendu .".'■""■ 

 Sess. Int. Congr. Geol. Washington, 1SU1, p. .'338, and ' The Age of the Igneous 

 Bocks of the Yellowstone National Park,' Am. Journ. Sci. ser. 4, vol. i. (1896J 

 p. 445. 



2 Weed, W. H., 'The Laramie and the overlying Livingston Formation in 

 Montana,' Pull. U.S. Geol. Surv. No. 105, pp. lli & 21, Washington, 18'J3. 



