Evidence of Joriuer Glacial Periods. 281 



dykes appeared to mark the radial fissures on which sporadic cones 

 (" puys ") were thrown up, after the great central volcanoes became 

 extinct ; and this view is supported by the circumstance of the close 

 analogies between the materials erupted at this later period, and the 

 rocks which constitute some of the undoubtedly Post-Mesozoic 

 dykes. 



Dr. Geikie supports his view, that the plateau-basalts of the 

 Western Isles of Scotland and of Antrim were formed by " fissure- 

 erui)tions," by facts which he noticed in the Snake-River country, 

 in the year 1879, while he was making an excursion to the Yellow- 

 stone Park, and also by observations made by Captain Button in 

 the Grand Canon country, in Utah, and in New Mexico. 



^Yith respect to Dr. Geikie's own observations, it was pointed out 

 that geologists who have had more time and opportunity for the 

 detailed study of the district in question, like Captain Reynolds, 

 Dr. Hayden, and Mr. Clarence King, all agree that there is abund- 

 ant evidence of ordinary volcanic action having occurred in the 

 Snake-River country ; and the last-mentioned author distinctly 

 points out the great paucity of dykes, and the absence of any 

 evidence of the existence of fissures such as those from which 

 " fissui'e-eruptions " are supposed to have taken place. 



Captain Dutton, although originally inclined to refer the lava- 

 fields of the Western Territories of the United States to "■ fissure- 

 eruptions," has, since his visit to Mauna Loa, and his study of the 

 floods of basalt that have flowed from that volcano, very candidly 

 confessed that, in view of these later observations, he is no longer 

 prepared to maintain his original position. 



If the effusive action taking place at many volcanoes be rightly 

 understood and appreciated — and the recent very interesting re- 

 searches of Prof. J. D. Dana in the Sandwich Islands have thrown 

 much new and important light on this subject — the theory of 

 " fissure-eruption " will be found to be as unnecessary as it is vague. 

 At some volcanic centres there is a preponderance of explosive 

 action ; at others the main result consists in the extrusion of lava- 

 currents ; while in most cases we find a combination of both kinds 

 of action. The Tertiary volcanoes of Scotland, like the existing 

 volcanoes of Iceland, are interesting as exhibiting evidence of both 

 the effusive and the explosive action on the very grandest scale. 



January 23.— W. T. Blanford, LL.D., F.R.S., President, 

 in the Chair. 



The following communications were read : — 



1. " On the prevailing Misconceptions regarding the Evidence 

 which we ought to expect of former Glacial Periods." Ey Dr. James 

 Croll, F.R.S., F.G.S. 



The imperfection of the geological record is greater than is usually 

 believed. Not only are the records of ancient glacial conditions 

 imperfect, but this follows from the principles of geology. The 

 evidence of glaeiation is to be found chiefly on land-surfaces, and 

 the ancient land-surfaces have not, as a rule, been preserved. 



