89 



rather than general causes ; but taken as a whole they may be said to vary 

 inversely as the silica. In the most siliceous rocks they make up only 4 or 5 

 p.c. of the mass ; in the least siliceous they sometimes form as much as 40 or 

 even 50 p.c. 



The alkalies, potash and soda, present very interesting relations. If we 

 except the leucite-, nepheline- and hauyn -bearing rocks, which are somewhat 

 limited in their range in space and time, and which, taken together, appear to 

 be insignificant in bulk when compared with all the other igneous rocks, then we 

 may state that, as a general rule, the total alkalies increase with the silica-per- 

 centage, and the relations of the two alkalies, potash and soda, become re- 

 versed ; soda is the dominating alkali in the basic, potash in the acid rocks. 

 It must be understood that this law is true only in a broad and general sense, 

 especially as regards the relations of the two alkalies. In rocks of the inter- 

 mediate class, the andesites and porphyrites and their granular representatives, 

 the more acid dolerites (diabases) and diorites, the mutual relations of the two 

 dominant alkalies often cease to have any very definite relation to the silica- 

 percentage. 



The leucite-, nepheline- and hauyn-bearing rocks, represented in England 

 so far as we know at present by the solitary example of the phonolite of the 

 Wolf Rock, are characterised by an abnormally high percentage of alkalies. 

 They are extensively developed in certain parts of the continent of Europe, in 

 some of the volcanic islands of the Atlantic, and sparsely in North America. 

 They have been recorded also from the north of Africa, Persia and the Sand- 

 wich Islands. It is worthy of note that where they bulk most largely the 

 normal igneous rocks, the andesites, basalts and rhyolites, are not very ex- 

 tensively developed ; and that where the latter occur in great abundance the 

 former are rare or altogether absent. In the Schemnitz district of Hungary/ 1 ' 

 for example, andesites are developed on an enormous scale, and basalts and 

 rhyolites to a considerable extent. Leucite-, nepheline- and hauyn-bearing 

 rocks are conspicuous by their absence. In the tertiary volcanic district of 

 Bohemia this condition of things is reversed. If we take a general view of 

 the igneous products of all geological periods and of all countries it seems 

 impossible to avoid the conclusion that the leucite-, nepheline- and hauyn-bear- 

 ing rocks are separated from the great bulk of igneous products by peculiari- 

 ties in their distribution both in space and time, as well as by peculiarities in 

 chemical and mineralogical composition. 



In preparing material for a bulk-analysis of any igneous rock care must 

 be taken to select a sufficient quantity to give a satisfactory average. In the 

 case of coarse-grained rocks it is necessary to take a very much larger quantity 

 than is actually required for analysis. Attention should also be paid to any 

 variations which may occur in the composition of the rock-mass. Plutonic 

 rocks often show a considerable amount of variability in the mineralogical 

 and chemical composition of different portions of the same rock-mass. Patches 

 often occur which contain the same minerals as the main mass of the rock, but 



(1) JUDD. On the ancient volcano of the district of Schemnitz. Q.J.G.S., Vol. XXXII., p. 292. 



