50 



have finished the work by removing what the oxygen had commenced to change. 

 Nearly all the denudation changes of the surface over that wide expanse of 

 country lying to the westward of where you now stand is due to atmospherio 

 agency, operating through long, long periods of time. Again, from this spot you 

 see what geologists call faults or dislocations ; you observe below you one of 

 these fractures and displacements which has determined the course of the "Wye, 

 and caused the river to find its serpentine channel around the Yat. Here, 

 therefore, let me say, before we leave this place to examine the erratic boulder, 

 you have — 1st, the agency of fire exemplified in the Malverns ; 2nd, the agency 

 of water fully displayed in the conglomerate beds of the Old Red Sandstone 

 and other sedimentary rocks ; and, 3rd, atmospheric agency demonstrated in 

 the enormous denudation which has taken place in all the red rocks, forming 

 the Devonian landscape before us (applaxise). 



The party then proceeded to the private grounds in the Yat to examine a 

 large boulder of hard yellow sandstone called "Pennant," which is here seen 

 resting upon Carboniferous Limestone, and has been brought into its present 

 position by some powerful agency, for it is clearly a rock mass that has been 

 transported from a distance, there being no such rock as that of which it formed 

 a part in the vicinity of Symond's Yat. 



Dr. Wright observed that the history of erratic blooks formed a won- 

 derful chapter in the fourth volume of the Rock Book, and the true explanation 

 of the force that had moved these enormous masses of rocks from great distances 

 and over planes, rivers, lakes, and seas had been reserved for the men who 

 were our contemporaries. Ice was the great transporting agent, and the old 

 Glacier in the valley, or the Berg in ocean were the means by which these trans- 

 ports were made. Perhaps, he said, I can best explain what I mean if I show you 

 the photograph of the Picrrc-d-Bot, kindly sent to me by my friend Professor 

 E. Desor of Neuchatel ; it is one of the many large erratic blocks that have 

 been transported from the chain of Mont Blanc on the surface of the large 

 glacier that once extended across the plain of Switzerland and shed its enormous 

 fan-shaped terminal moraine upon the slopes and valleys of the Jura mountains 

 overlooking the Swiss plain. This granite boulder is in length 53 feet, in 

 height 43 feet, and in breadth 20 feet ; it contains about 40,000 cubic feet of a 

 beautiful greyish granite, Protogine, of considerable hardness with large crystals 

 of feldspar of a milk whiteness, or tinted violet, united to crystals of quartz and 

 grouped between brilliant plates of greyish or blackish mica. This gneissic 

 granite is derived from the red Aiguilles of Chamouni which make part of the 

 massive structure of Mont Blanc, distant about 70 miles from its present resting 

 place on the flanc of the Chaumont, three miles from Neuchatel, and at an 

 elevation of 900 feet above the Lake : no force in nature but ice could have 



