ox SEISMOLOGICAL IXVESTIUAIIJ-V. " 185 



The distortions observed in fossils and pebbles, the difference in 

 thickness of contorted strata, and the ' creep ' in coal-mines, all indicate 

 that great pressures may set up movements in stratified materials corre- 

 sponding to a flow. Mr. AVilliam Barlow, in a paper on the ' Horizontal 

 Movements in Rocks,' ' as evidence of this, calls attention to the contor- 

 tions and foldings observed in glacial drift produced by a load above, the 

 dip seen on the face of the Grand Canon of Colorado, and the slight eleva- 

 tion observed in the area surrounded by cliffs known as the ' San Rafael 

 Swell.' These and other appearances may be regarded as instances of 

 'creep' upon a large scale, when materials have been squeezed out from 

 beneath superincumbent strata. 



In studying bradyseismical movement we usually take cognisance of 

 that which is most apparent. This is the vertical component of a dis- 

 placement, whilst the horizontal movement may be entirely overlooked. 

 The geotectonic structure of many countries, however, shows us that dis- 

 placements by horizontal thrust have taken place on an enormous scale, 

 and it is not unlikely that these forces, accelerated by the effects of crush, 

 are yet in operation round the basal contours of continental areas. Sub- 

 oceanic earthquakes are therefore announcements that sub-oceanic brady- 

 seismic action is in progress, and because these disturbances are more 

 numerous round the submerged frontiers of continental domes and in mid- 

 ocean than they are on land, it may be concluded that the distortions and 

 displacements due to bending, thrust, and crush are greater beneath the 

 sea than they are upon continents and islands. 



Eartltquakes and Landslides. — In addition to these bradyseismical 

 effects, which only produce appreciable changes in sub-oceanic contour 

 after the lapse of long intervals of time, there are the effects which accom- 

 pany the actual shaking, which we may assume are not far different from 

 those effects which we see produced by earthquakes originating on land. 

 Many earthquakes which we feel, although they may create alarm and 

 shatter chimneys, do not produce any effect upon rocks and cliffs. This, 

 however, does not preclude the idea that shakings of equal intensity would 

 not produce effects upon submarine slopes, where, as compared with similar 

 slopes on land, critical conditions may more nearly approach in character 

 to the mechanism of the hair trigger. Severe earthquakes on land are 

 almost always accompanied by great landslides, and mountains which may 

 for ages have been green with forest growth l)y the sliding away of 

 materials en their sides suddenly present the appearance of having been 

 whitewashed. The probable effect of similar shakings originating beneath 

 the ocean in the vicinity of steep slopes needs no explanation. 



Another effect which sometimes accompanies these disturbances, and 

 which may have been their cause, is the creation of a fault 50 or 150 

 miles in length, by which the country on one side of this, relatively to 

 that on the other, has been suddenly raised or lowered 20 to 30 feet. 

 Earthquakes of this nature, if of submarine origin, would naturally 

 produce similar effects over large areas, and, if the magnitude of the 

 displaced materials, whether by landslides or faulting, were large, as com- 

 pared witli the depth of the superincumbent waters, would also give rise 

 to sea-waves. 



One of the most recent examples of effects of this description was that 

 which occurred on June 15, 1896, off the north-east coast of Japan. On 



' Quart. Journ. Geol. Soc, November 1888. 



