264 E. B. Bailey — Drakes Island, Plymouth. 



and its igneous structure cannot be made out "with a pocket lens r 

 though quite clear under the microscope. On the other hand, many 

 of the fragments in the ashes B and E, especially E, when examined 

 "with a lens, show small felspars, set in a compact, sometimes almost 

 flinty, base. These fragments have not uncommonly a well-marked 

 fluxion structure, and under the microscope they look distinctly 

 more acid than the lava D. All the igneous products of Drake's 

 Island are very decomposed, with a plentiful development of albite, 

 calcite, etc. In the tuffs, amygdales filled with albite, anorthoclase, 

 and orthoclase, more particularly the first-named, occur in profusion. 

 This feature is probably correlated with the comparatively acid 

 nature of many of the enclosed fragments ; the lava D was not 

 closely examined for felspar-amygdales, but the similar rocks of 

 the mainland 1 are seldom well endowed in this respect. The 

 amygdales of the tuffs are interesting since they have in large 

 measure resisted the Post-Carboniferous movement. At the same 

 time it does not appear likely that the amygdales antedate the 

 formation of the tuff, since the latter is itself cut by veinlets of 

 alkali felspar. It is possible, however, that they formed before 

 the tuff cooled. 



Much the most peculiar rock on the island is an amygdaloidal 

 limestone, closely mimicking a lava in appearance. A suggestion 

 as to its origin is offered below (under C), where the various divisions 

 distinguished by letters on the map are described seriatim. 



Groups A-E. 



A. Seventy feet of bedded grey limestone with some chert. The- 

 limestone does not appear very fossiliferous, but is so covered with 

 barnacles that detailed examination is difficult. 



Junction A B. — Fault. 



B. One hundred and fifty feet of rather indistinctly bedded tuffs. 

 These contain somewhat frequent lumps of limestone, and occasional 

 separate corals, apparently blown into their present position. 



Junction B C. — Probably a fault. 



C. Fifty feet of limestone. The lower part has a little chert, and 

 is well bedded and fossiliferous, consisting in some exposures of 

 Stromatopora, with encrinites, etc. The upper part is practically 

 unbedded, and is often very vesicular like a lava. The vesicles are- 

 filled, either partially or completely, with quartz and calcite, the 

 former in finely crystalline aggregates. The appearance strongly 

 suggests that this portion of the limestone was ejected as a hot 

 bubbling calcareous ooze — a mud lava in fact. 



Junction C D. — Very clear upward passage from limestone to- 

 pillow-lava exposed on south-west coast just north-west of a well- 

 marked fault. The top of the limestone for a couple of feet serves as 

 a matrix more or less completely isolating highly vesicular pillows 

 of lava. The dip is very gentle. 



D. Two hundred feet of lava. The lava (or lavas, if there is more 

 than one flow) becomes compact a little above the vesicular pillowy 

 base just described. The compact zone is of considerable thickness,, 



1 E. H. Worth, op. cit., p. 225, pi. iii, fig. 13. 



