340 L. V. Pirsson — Geology of Bermuda Island ; 



and it is clear that they are lavas of lamprophyric character. 

 The relative amount of mafic minerals and felsic cement is 

 variable, and where the former is large the augitic types are 

 much like those placed under augitite by Rosenbusch. The 

 lamprophyric nature of many augitites is indeed commented 

 on by this writer.* The type, rich in biotite and without 

 augite, could not, however, be placed with them ; and those 

 who demand a separate name for the effusive rocks as con- 

 trasted with the intrusive ones, would object to calling it 

 ouachitite, which it is most like in composition and in genesis. 

 The name of bermudite is suggested for consideration, to obvi- 

 ate this difficulty. 



Chemical Composition. 



It has not been possible to obtain thoroughly satisfactory 

 material for an analysis of the Bermuda lava, partly because 

 none has been found in the samples that was not altered, as 

 previously observed, and partly because no definite solid mass 

 large enough to afford the requisite quantity of substance was 

 obtained. The fine granular samples are representative of the 

 lavas between certain levels, and where pieces large enough to 

 furnish sections occurred in them it was found, as previously 

 noted, that two types of rocks were commingled. Therefore, 

 there was no assurance in the analysis of one of the samples 

 that one definite rock substance was being investigated, as 

 would be the case when a solid piece of rock is available. 

 Consequently an extended detailed analysis did not seem war- 

 ranted, but, on the other hand, since the two rock types are 

 much of the same general nature and the melilite-basalt in 

 some is much in excess, it was thought that a partial analysis 

 would be of some interest and would afford additional light on 

 the nature of the lavas. This has been carried out by Mr. R. 

 C. Wells of the chemical laboratory of the U. S. Geological 

 Survey, to whom grateful acknowledgment is here made for 

 this assistance. The analysis is given in No. 1 of the adjoin- 

 ing table, and those of some related rocks are added for com- 

 parison. 



The analysis shows at once the character of the group of 

 alkalic lamprophyres. An attempt has been made to compare 

 it with the two kinds of rocks of which presumably the sample 

 was composed, melilite-basalts and monchiquitic rocks. There 

 are not many analyses of melilite-basalts available, and most of 

 these are not recent, and, therefore, not made according to 

 modern methods ; they are consequently more or less defective. 

 No. IV is given that of the typical rock from Hochbohl. The 

 * Massige Gesteine, vol. ii, p. 1475, 4th ed., 1908. 



