43 



Si0 2 



Ti0 2 



A1 2 3 



Fe 2 ; 



FeO 



CaO 



MgO 



K 2 



H 2 



99-82 99-84 100-16 100-04 99-68 



Sp. Gr. 2-640 



I. Augite-andesite. (1 > Arrnathwaite Dyke, Cumberland. 

 IA. Glassy base of same rock. 



II. Hypersthene-andesite.< 2 > Carhope. Cheviot District. 

 HA. Glassy base of same. 



HB. Devitrified base of hypersthene-andesite from Allerhope Burn, Cheviot 

 District. 



III. Hypersthene-andesite. Mount Shasta, California.' 8 ) 

 IIlA. Glassy base of same. 



IV. Dacite. Lassen's Peak, California. 

 Glassy base of same. 



A comparison of these analyses leads to the conclusion that the effect of 

 crystallisation in an igneous magma of andesitic composition is to increase 

 the percentages of silica and alkalies, and to diminish those of the other 

 constituents, in the part remaining liquid. Moreover, the effects as regards 

 the alkalies is to increase the potash relatively to the soda ; so that in 

 certain cases the relations of the two alkalies become reversed ; soda being 

 the dominant alkali in the rock, and potash, in the glassy residuum. The 

 general conclusions here reached by purely chemical considerations agree 

 with those deduced from an examination of the order of crystallisation of 

 the different rock-forming constituents as determined by the study of thin 

 sections under the microscope. (4) 



Various attempts have been made to express the facts known with regard 

 to the composition of the different kinds of igneous rock in terms of some 



(1) TEALL. Petrological Notes on some North-of -England Dykes. Q.J.G.S., 1884, p. 224. 



(2) PETEBSEN, J. Untersuchungen am Enstatite-porphyrit aus den Cheviot Hills. Inaug. 

 Diss. Kiel, 1884. 



(3) HAGUE & IDDINOS. Notes on the Volcanoes of Northern California, &c. A.J.S., 

 vol. XXVI., p. 222. 



(4) ROSENBUSCH. N.J., 1882, Band II., p. 1. See also TEALL. G. M. Decade III., vol. II., 

 p. 118. 



