age of Arthur's seat, Edinburgh. 147 



Such appears to be the series of events to which the rocks of 

 Arthur's Seat owe their origin. It will be interesting, in conclusion, 

 to notice the difference in the mode of preservation of the old volcanoes 

 of Central Scotland, and that of the numerous Tertiary volcanoes the 

 remains of which I have before described as existing in the Hebrides. 

 While the latter owe their survival simply to the hardness or power 

 of resisting denudation of the materials which compose them, the 

 former have been enveloped and protected through long periods in 

 great masses of sedimentary materials. It is through the removal 

 of the matrix of investing strata by denudation that the buried 

 igneous rocks have been once more exposed ; and we may therefore 

 justly speak of such vestiges as "fossil volcanoes." 



Briefly recapitulated, the results arrived at in the present memoir 

 are as follows : — 



1. The antecedent improbabilities of the theory of two periods 

 of eruption for the rocks of Arthur's Seat are exceedingly great, 

 if not altogether insuperable. 



2. The supposed proofs of a second period of eruption break down 

 upon re-examination. 



3. The theory was found by its sagacious author, as he pursued 

 his investigations among the rocks of Edinburgh and his compari- 

 sons with other volcanic districts, to be untenable. 



4. All the phenomena of Arthur's Seat are capable of being ex- 

 plained by a much simpler theory, which has, moreover, the advan- 

 tage of bringing them into exact harmony with the appearances 

 exhibited at many other points in the same district. 



Discussion. 



Mr. Evans remarked that the simplicity of Mr. Judd's explana- 

 tion of the phenomena seemed to commend it to the consideration 

 of geologists. 



Prof. Ramsay, complimented the author on the clearness and ele- 

 gance of his paper. He said that he had been long well acquainted 

 with the locality described, and had accepted the idea that there 

 had been two periods of eruption, although he had always felt that 

 there were great difficulties attending it ; and he was glad that this 

 old idea was now shown to be incorrect. With regard to what the 

 author had said of the lines of faults, he thought that the conglo- 

 merates of the southern flanks of the Highlands were unconformable 

 to the older metamorphic rocks. 



Mr. Evans inquired whether the evidence is clear as to the sheet 

 of dolerite which crops out at St. Leonard's Craigs not following the 

 direction of the stratification, but cutting through it diagonally. 



Mr. Drew inquired how much of the diagrammatic section was 

 from actual observation, and how much from inference. 



Mr. Whitaker asked how the basalt on the Lion's Haunch was 

 placed in its present situation. 



The Rev. T. G. Bonney said that the configuration of the Hunter's 

 Bog valley had always appeared to him rather to favour the theory 



Q. J. G. S. No. 122. m 



