L. V. Pirsson — Geology of Bermuda Island. 331 



Art. XXVII. — Geology of Bermuda Island ; Petrology of 

 the Lavas ; by L. V. Pirsson. 



Introductory. — The samples obtained from the Bermuda 

 well, described in the preceding number of this Journal, do not 

 afford material well suited for petrographic study. In the 

 zone of sedimentary deposit down to 700 feet frequent chips 

 and pebbles occur, sufficiently large to furnish thin sections of 

 the usual size, but this material has been greatly altered by 

 atmospheric agencies. In the zone below this, where firm 

 rock is encountered which, buried beneath the sea, has never 

 been exposed to the atmosphere, the drill reduces the rock to 

 the condition of a sand varying from fine to medium in size, 

 and only a few somewhat larger pieces occurred in the samples 

 studied by the writer. The difficulty is enhanced by the 

 extreme density, compactness, or fine grain of the rocks. 

 In order to obtain sections sufficiently thin to observe the 

 groundmass components it is necessary to grind them to thick- 

 nesses below 0*02 millimeter, which is not an easy operation 

 with small grains. The sections first prepared were those 

 made from the larger chips from the sedimentary zone and 

 from them it was thought that they were of altered augite- 

 andesite, as stated in the preliminary paper. * Further study 

 of the fresher material has shown that this was not correct and, 

 as will be seen, the lavas have a quite different character from 

 andesite. 



Sections have been prepared from nearly all the samples 

 and while they all yield some information, especially with 

 respect to the larger phenocrystic minerals, in a number of 

 them, cut with especial care as thin as possible by Mr. W. 

 Harold Tomlinson, areas occur which are fairly well suited for 

 the study of the very fine groundmass. In all the sections 

 this groundmass is more or less altered, and in most cases very 

 much changed. When the petrographic work was nearly 

 finished I received a letter from Dr. H. H. Thomas of the 

 Geological Survey of Great Britain, in which he stated that he 

 had also had the opportunity of studying a set of samples from 

 the Bermuda well, received by the Survey through the British 

 Colonial Office. From his statement it seemed that he had 

 found some better material and had therefore better sections 

 than the writer, and I requested him to send me the results of 

 his observations on the petrography of the lavas. This he has 

 most kindly done, and he has also added some microphotographs. 

 Dr. Thomas' letter sums up so well the petrographic characters 



*This Journal, vol. xxxvi, p. 70, 1913. 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIII, No. 226. — October, 1914. 

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