62 PEOF. J". W. JTJDD 01^ TEKTIAEY 



the evidence upon which I relied in arriving at my former conclu- 

 sions, with the aid of those important methods which in recent 

 years have been introduced into petrographical research through the 

 labours of crystallographers and chemists. 



Although the present memoir has been somewhat delayed in its 

 appearance by the necessity for considering and weighing the vast 

 amount of evidence which has been adduced upon the other side of 

 the question, by authorities whose position demanded the most careful 

 consideration of their views, yet I venture to hope that the delay has 

 enabled me to treat the question with greater thoroughness than 

 would have been possible if I had written at an earher date. The 

 present paper forms a sequel to that which I published last year on 

 the intimately associated peridotites of the same district. 



The views which I enunciated in 1874 and ]876 have recently 

 met with the strongest support from the observations recorded by 

 Messrs. Arnold Hague and J. P. Iddings, of the U. S. Geological 

 Survey. In their very remarkable monograph '' On the Development 

 of Crystallization in the Igneous Eocks of Washoe, ISTevada, with Notes 

 on the Geology of the District"*, they state that a careful micro- 

 scopic study of the extensive series of rocks collected by the survey 

 during the mapping of the district around the celebrated Comstock 

 Lode, has led them to the conclusion that the most highly crystal- 

 line rocks of the district are — like the associated andesite and other 

 lavas — of Tertiary age ; and they further show that highly crystal- 

 line varieties, like quartz-porphyry, quartz-diorite, diorite, and dia- 

 base, graduate respectively into rhy elite, dacite, andesite, and basalt. 

 The great interest of this investigation, and that which consti- 

 tutes its chief novelty, arises from the fact that the specimens upon 

 which the conclusions are based were obtained from artificial 

 excavations — the great Sutro Tunnel and deep shafts made for 

 mining-purposes around the Comstock Lode — while my own ob- 

 servations were made on specimens derived from rock-masses laid 

 bare by the natural agents of denudation. 



In no class of igneous rocks does the apparent distinction between 

 the older and younger types come out more strikingly than in those 

 of hasic composition. This is due to the circumstance that the 

 minerals of which these rocks are composed are, as a rule, less stable, 

 and therefore more liable to be converted into pseudomorphs, than 

 the minerals of either the acid or the intermediate class of rocks. 

 In the case of these basic rocks it is of especial importance to distin- 

 guish between the original minerals and those which are secondary. 

 Now the Scottish representives of the basic class of igneous rocks 

 are of especial interest to the petrologist on two distinct grounds. 

 In the first place we are able, as I have shown, to determine from 

 their present positions the approximate depth from the surface, and 

 the other conditions under which the several varieties must have 

 consolidated ; and, secondly, although they do not present us with ex- 

 amples of all the extreme results of alteration in the minerals of which 

 they are composed, they exhibit the incipient stages of most of these 

 * Bulletin of the U. S. Geological Survey, No. 17 (1885). 



