Notices of Memoirs — Prof. W. M. Davis — Dr. J. Milne. 467 



These facts indicate that granite has been one of the chief sources, 

 not merely of the arenaceous, but also of the argillaceous rocks. It is 

 not improbable that both may ultimately be proved to have been 

 derived from the siliceous acid layer believed by Durocher and 

 Haughton to have been the first to become solid in the cooling globe. 



YIII.— Glacial Eeosion" in North Wales.^ By Professor W, M. Davis, 



rpHE mountains of the Snow don district are believed to represent 

 JL a group of monadnocks which surmounted the peneplain to which 

 a large part of the region was reduced in Tertiary time. The valleys 

 between the monadnocks were somewhat deepened by normal erosive 

 processes, in consequence of a general elevation of the region in late 

 Tertiary time. As a result, the topography of the Snowdon district 

 in immediately pre-Glacial time -may be described as exhibiting 

 a group of well- subdued mountains, drained through valleys of 

 somewhat shai-pened form. The difference between the forms thus 

 described and the forms seen to-day in the Snowdon district is very 

 great, both in amount and in kind, and cannot be accounted for by 

 normal erosion during Glacial and post-Glacial time. But the difference 

 is, in amount and kind, just what might result from glacial action, if 

 it be postulated that glaciers are effective eroding agencies. The 

 depth of glacial erosion in certain cwms and valleys is believed to 

 have been 400, 600, or 800 feet ; the breadth of glacial erosion must 

 have been of even greater measure. 



IX. — The Duration" and Direction of Large Earthquakes.^ By 

 Dr. John Milne, F.R.S. 



SMALL earthquakes, as for example those which occur in this 

 country, have a duration of a few seconds near to their origin. 

 At places 50 or 100 miles distant they may not be recordable. The 

 duration, therefore, has varied between a few seconds and zero. "With 

 many large earthquakes, however, this decay during transmission is 

 not appreciable, and duration near to their antipodes may be as great 

 as it is near to their origin. Duration as one of these disturbances 

 travel, rather than decreasing, at times appears to increase. The 

 greatest duration is at about 90° distance from an origin. That 

 which occurs may be compared with what we observe after a flask 

 of water has been tilted. The contents oscillate like a pendulum, 

 and any one part of the fluid comes to rest about the same time as 

 any other part. 



Another observation in connection with recent seismological observa- 

 tions is that large earthquakes travel farthest in particular directions. 

 I have taken seventy-nine large disturbances with fairly well-known 

 origins south of the Caucasus, north of India, and to the east or south 

 of Japan. These earthquakes have travelled farther to the west than 

 to the east, and there has only been a small percentage of them that 

 have found their way across the equator to observatories in the southern 

 hemisphere. 



^ Eead before the British Association, Section C (Geology), Dublin, Sept. 1908. 



