Edinburgh Geological Society. 171 



Silurian beds of the Pentland Hills, by Mr. Jobn Henderson, who 

 also exhibited the well-preserved fragments which he had found of 

 this interesting, crustacean. 



The third ordinary meeting of this Society was held in the Lecture 

 Hall of the Museum of Science and Art. There was a numerous 

 attendance. Mr. M. Lothian occupied the chair. 



The Chairman introduced Mr. Archibald Geikie, F.E.S., F.Gr.S., 

 who proceeded to deliver a lecture upon " Geological Time." 



Mr. Geikie spoke of the geological evidence of the earth's an- 

 tiquity under three heads : — 1, Inorganic evidence ; 2, Organic 

 evidence ; and 3, The bearing of astronomical and phj'^sical data 

 upon the question. Under the first of these divisions he noticed the 

 fact that the whole surface of the earth, from the mountain-top to 

 the sea- shore, was slowly changing and undergoing a process of 

 waste. The process of waste, however, v/as so slow as scarcely 

 to be appreciable in a man's lifetime. Besides these changes, 

 there was the action of icebergs and glaciers, which acted 

 mechanically in grinding down the rocks to sand and mud. The 

 lecturer illustrated this portion of his lecture by reference to well- 

 drawn pictures of the glacier-fields of Norway, and remarked that 

 in every Scottish glen they found that the rocks had been ground 

 down in the same way as they were being planed away in Norway, 

 and how vast must have been the time taken in the process. Eepre- 

 sentations of sandstone rocks standing upon the Laurentian gneiss 

 in Sutherland and Eoss-shires were pointed to as illustrating the 

 gradual manner in which the former had been worn, and proving 

 that there had not been any sudden convulsion or cataclysm in pro- 

 ducing the geologic changes alluded to. Coming to the organic 

 evidence of the changes, the lecturer referred to the occurrence of a 

 Germanic Flora in this country as a proof that the temperature was 

 much more moderate in this country now than it had been in former 

 eras. Plants that formerly bloomed in our valleys had now become 

 alpine in their character, and were only to be found on the tops of 

 the hills. In regard to the Fauna, shells that had lived during the 

 Glacial period were still to be found in our seas, and only a few 

 species had become extinct. As the plants had become inhabitants 

 of hill tops, so these shells of a former age had become denizens of 

 the deeper parts of the sea. All these changes had been effected 

 since the Glacial period, and when they knew, as they did, the length 

 of time that had elapsed since then, how long must have been the 

 time since the plants and animals of former periods were extin- 

 tinguished ! In regard to the bearing of astronomical and physical 

 data upon the question, Mr. Geikie remarked that the geological 

 record afforded no data for computing the length of its periods in 

 years. If, however, it contained traces of any great cosmical event 

 it might be possible to arrive at the date of such an event, for the 

 astronomical periods were not like those of geology, but could be 

 computed in years. It had long been the belief of many geologists 

 that if any actual data of this kind were to be found, it must be 



