3±2 L. V. Pirsson — Geology of Bermuda Island ; 



higher content in magnesia, while the lime is about equal or 

 even exceeds.* The more pure melilite types, judging from 

 No. IV and from No. VII, which is that of the intrusive rock, 

 alnoite, and from other analyses as well, are distinctly lower 

 in silica than those which are melilite-nephelite basalts, though 

 not otherwise essentially different. The Bermuda analysis is 

 more like II and III than IV, but differs from these in the 

 higher alumina and lower magnesia. The former explains the 

 considerable amount of analcite present, which in this rock 

 represents the nephelite without regard to whether the anal- 

 cite be considered original or secondary, while the lower mag- 

 nesia helps to. account for the rather small amount of olivine. 

 The lime of the Bermuda rock, as mentioned later, is con- 

 sidered essentially original. 



Two analyses, Nos. V and YI, of those rocks of the monchi- 

 quite group, which are most like the type found in the sam- 

 ples, are also included in the table. It will be seen that they 

 have much the general characters of the Bermuda sample, 

 the most marked difference being that one has higher magne- 

 sia and that they are somewhat lower in alkalies, especially 

 potash. It may also be noticed that they are much like the 

 nephelite-melilite-basalts, the most notable feature being the 

 higher alumina. 



The inference to be drawn from the analysis of the Bermuda 

 sample is, then, that it is what we should normally expect from 

 a mixture of the two determined types; but the table shows 

 also that it does not give data from which one can judge as to 

 the relative quantities of them present. 



Quantitative classification and Norm. 



It was thought that it might be of some interest to calculate 

 the norm of the above analysis and the determination of its 

 position in the Quantitative System ; it is understood, of 

 course, that since the rock material is altered, composed of two 

 types, and the analysis a partial one, too much importance 

 most not be attached to any facts or relations yielded by this 

 procedure. In making the calculation it is necessary to con- 

 sider the lime, which is in combination with calcite, a second- 

 ary mineral. Soellnerf argues that the lime in the secondary 

 calcite in melilite-basalts has not been brought in from outside, 

 but has come from the alteration of the melilite, and should, 

 therefore, be considered as original, and the writer believes 

 that this is true in the case of the Bermuda lava. The norm has, 

 therefore, been calculated ; A, on the assumption that sufficient 

 lime to satisfy C0 2 should be deducted ; and B, that it is all 

 original, and Ti0 2 is turned into perovskite. 



*Elemente der Gesteinslehre, 3d edit., p. 465, 1910. 

 f Loc. cit. 



