W: JV. Benson — Sjjilite Lavas, etc., in N.S.Wales. 17 



the rivers were again able to re-excavate valleys through the outwash 

 deposits ; terraces and plateaux of these gravels now overlook the river 

 courses. 



Penck ^ has given an excellent description of the erosive action of 

 glaciers, showing how the recognized features of glaciated mountain 

 regions have been produced. 



Concerning the duration of the Glacial Period as a whole an 

 interesting diagram is given by Penck & Bruckner,* the main features 

 of which appear in Fig. 1. It must be remembered that many 

 geologists have been induced to consider the Glacial Period of short 

 duration because calculations based on the temperature gradient of the 

 earth's crust appeared to show that the habitable age of the earth 

 was between fifty and one hundred million years. But the discovery 

 of radium and its heating effects have served to show that this view 

 was based upon incomplete data. We may, therefore, give full weight 

 to geological evidence when dealing with the age of deposits or 

 periods. Penck & Bruckner do not contend that they have done 

 more than make an estimate, and, although their estimate may be 

 either too great or too small, it has been made with a very full 

 knowledge of the Alpine phenomena. 



The scale in metres at the end of diagram, Pig. 1, gives the height 

 to which the snow-line was raised or lowered as a result of the changes 

 of temperature, whilst the scale at the bottom shows the estimated 

 age in years. 



During the retreat of the ice of the Wiirm Period pauses and 

 readvances took place. Three of these stages, the Daun (<?), 

 Gschnitz (g), and Biihl (J), shown on the diagram, have been 

 clearly recognized in many localities. Above the diagram, the position 

 of several of the stages which marked the advancing culture of man 

 is shown. 



A large number of moraines, showing halts or readvances of the 

 "Wisconsin ice-sheet in North America, have been mapped and described 

 among others by Leverett. 



VI. — Spilite Lavas and Radiolarian Pocks ]n New South Wales. 



By W. Noel Benson, B.Sc, Emmanuel College, Cambridge. 



rj^'HE very interesting remarks made by Messrs. Dewey and Plett 

 J on " Some British Pillow-lavas and the Rocks associated with 

 them " ^ have further emphasized Dr. Teall's observation of the 

 intimate association of Radiolarian rocks with lavas of this character, 

 and which is particularly well developed in Europe in the later 

 Devonian formations. It is further shown that such rocks are 

 characteristic of areas of continuous off-shore subsidence, and are 

 often intimately associated with intrusions of dolerite, quartz dolerite, 

 gabbro, and serpentine. 



Perhaps no occurrence of Radiolarian rocks is more often cited than 



^ Journal of Geology, vol. xii, pp. 1-19, 1905. 

 " Die Alpen in Ciszeitalter , vol. iii, p. 1168. 

 ■■ Geol. Mag., 1911, pp. 202, 242. 



DECADE V. — VOL. X. — NO. I. 2 



