422 THE MICROSCOPIC STRUCTURE OF VOLCANIC ROCKS. 



Slates," when last seen to the S.E., was about "W.NVW., which corre- 

 sponded nearly with the Charnwood strike. Hence he thought it 

 possible that they might be of the same age. 



Mr. Judd remarked that the circumstances connected with the 

 mode of occurrence of certain very important rock- constituents, such 

 as leucite, haiiyne, nosean, mellilite, &c, seemed to point to the 

 conclusion that species of minerals may present peculiarities of dis- 

 tribution, both in space and time, analogous to those of animal and 

 vegetable species. He suggested the necessity of constantly keeping 

 in view such a possibility when making comparisons between ancient 

 and modern volcanic rocks. 



Mr. Blanford inquired whether the ancient traps are not distinct 

 in their origin from true lavas. In India large districts are formed 

 of volcanic rocks much more recent than those of Wales, and pro- 

 bably of different composition. These Indian flows are of great 

 horizontal extent, but of small thickness. 



Mr. Porbes stated that the Indian rocks in question which he 

 had examined were not distinct from our basalts in composition 

 and structure, which showed that they were not poured out under 

 water. 



Mr. "Ward replied as follows : — To Mr. Eutley, that the flow 

 described as occurring in some of the altered ash-rocks was not 

 a decided crystalline flow, but one merely of chloritic material 

 around the larger fragments, and frequently along the bedding 

 planes. To Mr. Forbes and Mr. Koch, that the analyses brought 

 forward had been made from specimens carefully collected in the 

 field, and might be considered as representing the average chemical 

 composition ; the author used the word ash as denoting all material 

 shot out from a volcano ; in this case the ashes were mostly sub- 

 aerial. To Mr. Bonney, that although the analyses of the Cum- 

 berland lavas were very similar to those of porphyrites, the micro- 

 scopic structure of the latter, as described by Zirkel, did not at all 

 correspond with that of the Lake-district rocks. To Mr. Blanford, 

 that the Cumberland contemporaneous traps were as much entitled 

 to the name of lavas as any modern Yesuvian flow. 



