﻿October 4th, 1 go ./.] PROCEEDINGS. vii 



So far as herbarium examples furnish the means of com- 

 parison, it would appear that the Heaton Mersey plants are more 

 vigorous and ample in their growth. The chief difference is 

 seen in the length of the siliques or long pods, which are from 

 an inch to an inch and a quarter long, or half the normal length 

 of continental examples. As will be seen from the specimens 

 now submitted to the members, the fruiting heads are con- 

 spicuously flat topped. The seeds are produced in profusion 

 and, from the mounted seedlings before the meeting, may be 

 considered fertile ; they are minute and readily transported by 

 the wind. Now that attention is called to the existence of the 

 species in our flora, its occurrence may be looked for along the 

 course of the river, and elsewhere. 



Sir William Bailey then occupied the Chair, whilst the 

 President read a paper entitled " Note on the Effect of 

 Relaxation of Pressure in causing Folds at the Bottom 

 of Valleys," in which he called attention to a new cause of fold- 

 ing of the rock other than that which has been long recognised by 

 geologists as ultimately due to the folding of the outer layers of 

 the earth as they follow the contracting nucleus. The deep cuts 

 made through valleys to make watertight barriers in the con- 

 struction of reservoirs, revealed the fact that the bottom of the 

 valleys, wherever it was formed of shales and thin sandstones, 

 was more or less folded and contorted. These folds and con- 

 tortions caused the shales to let the water through with more or 

 less freedom, and he had been called in repeatedly to advise as 

 to how far it was necessary to carry the puddle trenches down 

 below the valley bottoms. He found, as a matter of experience, 

 that these folds w r ere superficial, and if the sinking were made to 

 a sufficient depth below the bottom of the valley, they disappeared 

 altogether. It was therefore obvious that they were not due to 

 deep-seated movements of compression, resulting from the con- 

 traction of the earth. They are due to the relaxation of pressure 

 caused by the removal of the rock by denudation from the area 

 of the valley, and are analogous in every particular to " the 

 creep" in coal workings, caused by the excavation of coal, by 

 which the surrounding strata crush down into the area of relaxed 



