Notices of Memoirs—Dr. M. Manson on Gilaciations. 419 
Taste VIII. 
Barchoualees Mean length of focus 
ee gee 4 in miles. 
Strong ‘ : 124 
Moderate. a5 13 
Slight,a@ . A 12 
ee : | 4} 
Slight earthquakes are obviously divisible into two sub-classes, the 
first in which the focus is 9 miles or more in length, the second 
in which the focus is 6 miles or less in length.! The above Table 
gives the reason why the sound should be of nearly the same 
character in strong, moderate, and the first division of slight earth- 
quakes. Thus, the intensity of all but the slightest earthquakes 
depends, not on the magnitude of the focus, so much as on the amount 
of relative displacement along the surface of the fault. 
British earthquakes, according to the nature of the shock, are 
divisible into two classes, simple and twin, and this classification 
corresponds to a difference in origin. Simple earthquakes are due to 
continuous slips, as a rule along strike faults; twin earthquakes are 
caused by rotation of the median limb of a crust-fold along a transverse 
fault, the two foci coinciding with the crest and trough of the fold, 
the interfocal region with the practically undisplaced portion of the 
median limb about which the rotation takes place. In this connexion, 
it is worthy of notice that the average distance between the epicentres 
of British twin earthquakes is 10 or 11 miles, which agrees closely 
with the average distance of 9 to 12 miles between the crests of the 
great crust-folds in France. 
The average length of the focus in twin earthquakes is about 12 miles. 
Thus, the average length of focus in twin earthquakes and in other 
earthquakes, whether strong or moderate or, in certain cases, slight, 
is nearly the same, and is probably equal to, or it may be slightly 
more than, the average distance between the crests of the crust-folds. 
In other words, the magnitude of the crust-folds seems to determine 
the length of fault-slips along strike faults as well as along transverse 
faults. 
NOTICES OF MEMOTRS. 
—————— 
1.—Tue Srenrricance or Harty AND OF PLEISTOCENE GULACIATIONS.? 
By Marsprn Manson, Ph.D. 
fY\HE objects of this paper are to point out the significant differences 
between the preceding, accompanying, and succeeding phenomena 
of early glaciations and the corresponding phenomena of Pleistocene 
1 In determining the average for the second sub-class, a large number of very 
slight shocks were unayoidably omitted ; their inélusion would, of course, lower the 
average considerably. 
* Being an abstract of a paper read before the Eleventh Session of the Int. Geol. 
Congress, Stockholm, August, 1910. 
