HuTTON. — The Earthquake in the Amuri. 289 



ing to apply it to our earthquake it will be useful to explain 

 the theory rather more fully. 



Rock, of all kinds, is a more highly elastic material than 

 alluvial gravel or sand, and when an earth-wave passes from 

 rock into alluvium it will, unless it be perpendicular to the 

 plane of junction of the two formations, be partly reflected 

 downward and partly refracted towards the perpendicular to 

 the plane of junction (PI. XYII., fig. 1). If, however, the 

 direction of the wave was very oblique to the plane of junc- 

 tion, the whole wave might be reflected dowai into the earth, 

 and no shock would be felt on the alluvium (PL XVII., fig. 2). 

 On the other hand, when the wave passes from alluvium into 

 rock the refracted portion will be bent away from the perpen- 

 dicular to the plane of junction, and the reflected portion will 

 have its angle of emergence increased (PI. XVII., fig. 3) ; but 

 if the angle is small between the direction of the wave and the 

 plane of junction, then total reflection of the wave in an up- 

 ward direction will take place (PI. XVII. , fig. 4). This upward 

 reflection might be in the same azimuth as the direction of the 

 earth- wave, but inore commonly the wave wall be diverted to 

 the right or left according to the inclination of the plane of 

 junction. It is only the cases of total reflection that need 

 be considered here. 



The slopes of old valleys covered up wdth alluvium vary 

 very much; but, as the earth-w'ave is always more or less emer- 

 gent, the angle formed by the wave with the plane of junction 

 on entering alluvium w^ill generally be greater than the same 

 angle when the wave is leaving alluvium : consequently, total 

 reflection will be rare where the wave enters an alluvial plain, 

 but will be common where the wave leaves it. A glance at 

 Plate XVII. will explain this. It follows, therefore, that 

 along those margins of alluvial plains where the rocky slopes 

 face the origin of the earthquake the shock may be doubled or 

 trebled in force ; while along those margins w^here the rocky 

 slopes are turned away from the origin the shock will either 

 be normal or will be diminished in intensity. This does not 

 apply to a narrow valley, for in that case the whole contents 

 of the valley would be forced to vibrate as one system with its 

 rocky w-alls, and there would be neither refraction nor reflec- 

 tion. I will now try to apply these principles. 



Jack's Pass Hotel (31 miles from the epicentrum), Jollie's 

 Pass Hotel (34 miles), and Culverden Station (-34 miles) 

 are in narrow valleys, and would receive the normal shock 

 only. 



Balmoral (30 miles) and Montrose (32 miles) are on 

 alluvial plains near the margin where the wave entered 

 alluvium from rock, and consequently the shock in these places 

 was probably normal also. 



