214 Geological Society. 



Yoredale Shales, and lava plays only a subordinate part. In the 

 Miller's-Dale area the upper lava is the thicker, and extends over 

 a greater district than the lower, while in the Matlock area the 

 converse is true. In the former area the lavas are separated by 

 about 150 feet of limestone, in the latter by about 80 to 100 feet. 

 The upper lava of Miller's Dale is on a lower horizon than the 

 lower lava of Matlock, and the limestone above it contains at least 

 two bands of interbedded tuff. The lavas are vesicular and aruyg- 

 daloidal in structure, and often very much decomposed. They 

 contain olivine, augite, and felspars, magnetite and iron-oxide ; the 

 felspars are often present in two* generations. The sills are, for the 

 most part, ophitic olivine-dolerites, and pass from a very coarse- 

 grained dolerite through the intervening stages into a fine-grained 

 dolerite or basalt : they are similar in structure to certain Tertiary 

 dolerites. The following vents are described : — In the north- 

 western area, those at Speedwell, Monks Dale, and Calton Hill ; in 

 the south-east, at Cracknowl, the Grange-Mill vents, Ember Lane, 

 Moor Lane, and the Hop ton vent; in the south-west, Kniveton-Wood 

 Cottage, Woodeaves, and Wibben Hill. The majority of the vents 

 are composed of volcanic agglomerate ; but the Calton vent, near 

 Miller's Dale, is a typical basalt with a small portion of agglomerate, 

 and the Hopton vent is a breccia of basalt-fragments. The Toad- 

 stones have all been mapped on the 6-inch scale, and petrologicnl 

 accounts of the different rocks are furnished. 



2. ' Data bearing on the Age of Niagara Palls.' By Prof. Joseph 

 William Winthrop Spencer, A.M., Ph.D., E.G.S. 



The author has been engaged in investigations for a monograph 

 on Niagara Falls, to be published by the Geological Survey of 

 Canada. Soundings at all the points of great changes in the Gorge 

 have been successfully undertaken, borings were put down for the 

 exploration of buried valleys, and instrumental surveys made of the 

 original river-banks and the physics of the stream. The mean 

 recession of the crest-line of the Falls is found to be 4*2 feet a year 

 under existing conditions, and this rate has approximately obtained 

 for 227 years. But this rate will not give the age of the Falls, on 

 account of former great variations in the volume of the river and in 

 the height of the Falls themselves. The chief change in volume of 

 water depends on the fact that originally Lake Erie alone was 

 discharged over the falls, when the supply of water was only 

 35 per cent, of the present discharge. Lake Ontario, too, stood at a 

 higher level, and thus the cutting-back from Qneenstown to Foster's 

 Flats was effected with a small water-discharge and, at first, a low 

 head. After an uplift which raised the crest of the fall considerably 

 above Lake Ontario, a slight depression followed which ' drowned ' 

 part of the lower gorge. This cutting is calculated to have taken 

 15,500 years for a distance of 14,400 feet. Above Foster's Flats the 

 sudden widening indicates the inflow of the other lakes into Erie, 

 greater water-discharge, and greatly-increased rapidity of recession. 

 The changes in height of the Falls and resistance of the rocks are ex- 

 amined in detail, and the small influence of pre-Glaeial filled channels 



