616 PROF. J. P. IDDINGS ON EXTRUSIVE [Nov. 1 896, 



the most natural causes for which disturbance seem to be changes 

 of temperature, and possibly of pressure. And we should expect 

 the most marked results where these changes were the greatest. 

 We should expect to find the greatest changes for a given period 

 of time experienced by magmas introduced into the cooler parts 

 of the earth's crust ; it being understood that differentiation will 

 take place the more readily, the more liquid the magma ; and take 

 place, if at all, before the magma solidifies. We should then expect 

 a differentiation in the most deeply-seated magma to progress the 

 most slowly, other things being equal ; and further that it would 

 be less likely to reach such an extreme as that attained by a magma 

 nearer the earth's surface. But the chemical nature of the original 

 magma must have a marked influence on the results, and it seems 

 to the writer that magmas richer in alkalies are more readily differ- 

 entiated, and produce more extreme results. 



The suggestion of Prof. Pirsson x that at a volcanic centre the 

 hotter part, being at the centre of the reservoir, would be the locus 

 of the more siliceous products of differentiation, while the cooler 

 margin would contain the least siliceous products, has a direct 

 bearing upon the order of succession of eruptions within the centre 

 of a volcanic core, and is applicable to the Crandall volcano. 



The elaboration of the processes of differentiation, and of the 

 eruption of the resulting magmas, necessitates many considerations 

 involving the size, position, and shape of the reservoir, and possible 

 differences of temperature ; the times of eruption,, size, and shape of 

 the conduit, and the laws controlling the flow of liquids. For, with 

 varying viscosities in the several parts of a differentiated magma, 

 the relative liquidity of the parts will influence the order of eruption 

 according to the size of the conduit and the velocity of the flow. 

 The operation of so many factors must prevent uniformity in the 

 results of volcanic activity, but may permit numerous repetitions in 

 different localities of closely analogous series of results. 



If there be fundamental or primary as distinguished from 

 subsidiary processes of differentiation, and if the former be con- 

 sidered to affect the most deeply-seated magmas, while the subsidiary 

 processes take place in smaller reservoirs of magma nearer the surface 

 — and if these processes find expression in the magnitude of the bodies 

 of derived magma, and in the order of their eruption — then some 

 idea of the relative order of those processes which gave rise to the 

 many bodies of igneous rocks that constitute so great a feature in 

 the geology of the region just described may be gained if we climb 

 to the summit of a commanding peak in the Teton range and look 

 about us over the surrounding country. Standing, as we may be, 

 at an elevation of 13,000 feet above sea-level, and 7000 feet above 

 broad valleys to the east and west, with the Teton Mountains 

 descending rapidly below us to the north, we have an uninterrupted 

 view of the region of volcanic activity. Directly east the remnants 

 of the once lofty chain of andesitic volcanoes rise in a range of 

 rugged mountains, sombre in the colour of their barren crests, which 



1 Pirsson, L. V., ' Complementary Bocks and Eadial Dykes/ Am. Journ. 

 Sci. ser. 3, vol. 1. (1895) pp. 116-121. 



