anxiveHsaey address of the president. 167 



volcanic activity include two other series of intrusive masses, one of 

 a basic, the other of an acid nature. Through some of the basalt- 

 ])lateaux rise huge bosses of olivine-gabbro and allied rocks. That 

 these great piles of material are of later age than the bedded basalts 

 around them is shown by the way in which the gabbro has been 

 injected between and across the bedding of the basalts. They pre- 

 sent remarkably coarse granitoid textures, and in many places 

 segregations of the several component minerals, indicating the 

 gradual cooling and crystallization of the rock at some considerable 

 distance below the surface. No conclusive proof has yet been found 

 that any of the gabbro bosses established a connexion with the 

 surface and gave forth superficial ejections. They probably rose 

 in some of the vents which^had served as funnels for the outflow of 

 the older plateau-basalts ; but, instead of finding egress upward 

 through the deep pile of volcanic sheets, they seem to have been 

 thrust between the bedded lavas in the form of innumerable sills 

 and veins. The structure thus produced was similar to that of the 

 ' laccolites ' of the Henry ]SIountains, so well described by Mr. 

 Gilbert.^ The central portions of the bosses remained mainly 

 amorphous, and show now the coarsest texture. Owing to ex- 

 tensive denudation, the gabbro has had its cover of overlying rocks 

 stripped off, and has been deeply trenched into corries and glens, 

 between which the black rock mounts up into peaks and crests of 

 singularly jagged and picturesque forms. 



The other series of eruptive masses is strongly marked ofi" from 

 the gabbros alike in external form and internal composition. The 

 rocks in it are thoroughly acid, ranging from a vitreous condition 

 (pitchstone) through various trachytes, felsites, quartz-porphyries 

 and granophyres into true granites. Most of them are intrusive. 

 They occur in bosses, sills, and dykes or veins. As bosses they 

 form in Skye extensive groups of conical mountains like the domite 

 puys of Auvergne. Elsewhere they have been injected as sills 

 between the bedding of the Jurassic rocks, as in llaasay and Strath, 

 or in veins across the basalts of the plateaux and the gabbros. There 

 can be no doubt that they are the last of all the Tertiary volcanic 

 series, except the latest basalt-dykes which traverse them.'' 



^ ' Geology of the Henry Mountains/ by Gr. K. Gilbert, U.S. Geographical 

 and Geological Survey of the Rocky Mountain Region, 1877. 



^ An interesting corroboration of this sequence occurs in Iceland, where 

 the granophyres likewise pierce the basaltic lavas: C. W. Schmidt, Zeitsch. 

 Peutsch, Geol. Ges. vol. xxxvii. p. 738 ; Thoroldsen, Bihang t. Sv. Vet.-Akad. 

 Handl. 14. ii. (188S\ 17. ii. (1891) ; H. Btickstrom Geol. Foren. Stockholm, 

 Forhandl. vol. xiii. (1891) p. 037. 



