Geology and Mineralogy . 163 



II. Geology and Minekalogy. 



1. The Tertiary Volcanoes of the Western Isles of Scotland ; 

 by Pkof. John W. Judd, Q. J. Geol. Soc, Jan. 9, 1889. — A 

 brief notice of this important paper by Prof. Judd, prepared from 

 an abstract of it in the Geological Magazine of February last, is 

 published on page 412 of the last volume of this Journal. The 

 following are additional facts from the paper. Prof. Judd states 

 that his memoir of 1874 was the outcome of five years' of personal 

 investigation, and that the region has since been his frequent 

 field of study. The present paper reviews the opinions held be- 

 fore 1874, commends strongly the conclusions of Macculloch with 

 reference to the region, points out the divergences of later writers, 

 and then the results of his own researches, confirming in the main 

 Macculloch's views. One of the most important points brought 

 out in that paper was the earliest establishment of the fact that 

 "in a particular area there exists a complete transition of granitic 

 into glassy rock, both in the acid and basic series," that is, " of 

 granites and gabbros into pitchstones and tachylytes," thus estab- 

 lishing the true relation of the so-called Plutonic rocks to the 

 Volcanic" — making it in that case a relation of identity as regards 

 age, and of difference, but only small difference, in conditions of 

 crystallization. The conclusion was then doubted by most geol- 

 ogists, and rejected by many; but now it is adopted by Prof. 

 Geikie in his memoir on the same region and has gained general 

 acceptance. 



In the same paper of 1874, Prof. Judd presented the conclusion, 

 now sustained by Prof. Geikie, that there were five well-marked 

 centers of eruption — namely, Mull, Ardnamurchan, Rum, Skye and 

 St. Kilda — where the eruptions were on a vast scale, attaining 

 great thickness, with great numbers of dikes; "the largest of 

 these intrusive masses, especially the nearly horizontal sheets, 

 consist of gabbro and granite, while the smaller ones, the dikes 

 and the peripheral portion of the great bosses and sheets, pass 

 into dolerites, basalts and 'felstones' that are exactly similar to 

 the materials of the lava-currents;" that "the basic inti'usions 

 tend to form wide-spread sheets, while the acidic ones assume 

 those more bulky and lenticular forms for which the name 'lac- 

 colites ' has since been proposed by Gilbert." 



The Tertiary age and subaerial origin of the " Plutonic " and 

 Volcanic rocks were shown in the paper of 1874, and these points 

 also have been sustained by Prof. Geikie in his memoirs, the 

 facts proving that " many of the rocks occurring among the Ter- 

 tiaries present all the characters which would, if found among 

 older rocks, cause them to be classed as ' porphyrites,' ' rnela- 

 phyres,' ' diabases.' " 



Prof. Judd does not admit that the evidence points to the con- 

 clusion that the acidic rocks of the Western Isles are of younger age 

 than the more basic ones, and now holds that the interval between 

 the acidic and basic outflows has no special importance as a period 



