236 Reports and Proceedings — Geological Society of London. 



hornblende and viridite, while the felspar becomes cloudy, and 

 finally merges into a granular base. The crystals of augite are 

 often gradually replaced by an assemblage of felted microlites ; in 

 other cases their outlines are preserved, whilst their substance is 

 replaced by hornblende, the rock being thus converted into a uralite 

 dolerite or uralite diabase. When these rocks do not contain augite, 

 and are to a great extent composed of long bacillary hornblendic 

 crystals made up of parallel belonites, the ends of which are fre- 

 quently curved outwards, it is probable that hornblende was an 

 original constituent of the rock, which is therefore a true diorite. 

 Slaty or schistose greenstones are less frequent than in the western 

 districts, but on St. Cleer Down the " hornblende-slates " graduate 

 imperceptibly from crystalline dolerites into clay slate ; these are 

 not improbably of igneous origin. Some of the slaty blue elvans 

 are identical in chemical composition with the dolerites, and may be 

 highly metamorphosed ash-beds, although, from some of their cha- 

 racters, it seems more probable that they are true igneous rocks. 

 The felspar in the brecciated slates is almost entirely plagioclase. 

 and is derived from the disintegration of greenstones. With regard 

 to the age of the rocks described, the author states that they are 

 generally older than the granite ; for the vesicular lavas and many 

 slaty hornblendic bands are evidently contemporaneous with the 

 slates among which they are bedded, while the latter are often dis- 

 placed by the granite or traversed by granitic veins ; and, further, 

 the eruptive doleritic rocks which break through the sedimentary 

 beds never traverse the granite, but are often interrupted by it. 



3. "The Kecession of the Falls of St. Anthony." By N. H. 

 Winchell, Esq. Communicated by J. Geikie, Esq., F.E.S., F.G.S. 



The author's purpose in this paper was to arrive at an estimate of 

 the date of the last glacial period from the rate of recession of the 

 Falls of St. Anthony, near the junction of the Minnesota and Mis- 

 sissippi rivers. He stated that the country is covered with deposits 

 of glacial origin, that between the present falls and Fort Snel- 

 ling, a distance of eight miles, the existing river-gorge has been 

 formed since the deposition of the newer Boulder-clay, and that the 

 old river- valley is filled up with glacial deposits. The gorge is of 

 very uniform character, being cut through hard limestones resting 

 on soft sand rock, both lying quite horizontally. The country was 

 settled in 1856, and the recession of the falls has since been very 

 rapid, its rate having been accelerated by the erection of saw-mills, 

 dams, etc. From the accounts of various travellers who have visited 

 the falls in the last 200 years, the author endeavoured to obtain an 

 estimate of the true rate of recession. Between the visit of Father 

 Hennepin in 1688 and that of Carver in 1766 he finds a recession at 

 the rate of 3 "49 feet annually ; between Carver's visit and 1856 a 

 mean annual recession of 6'73 feet ; and between Hennepin and 

 1856 one of 5-15 feet. The time-estimates for the cutting of the 

 gorge would be, according to the above means, 12, 103, 6, 276 and 

 8, 202 years. The author considers the data upon which the second 

 of these numbers is founded the most reliable. 



