484 



rials varying in hardness and pliability. It is much to be desired that 

 scientific travellers who visit countries shaken by earthquakes would 

 observe with minute care all the phenomena attending the Assuring 

 of rocks and buildings. I have been informed by an eye-witness of 

 one of the late minor earthquakes in Chili, that the walls of his 

 house were rent vertically, and made to vibrate for several minutes 

 during each shock, after which they remained uninjured and without 

 any opening, although the line of the crack was still visible. On 

 the floor, at the bottom of each rent, was a small heap of fine 

 brjckdust, evidently produced by trituration. In such instances it 

 would be desirable to obtain fragments of the rent building, and 

 to compare them with the walls of natural fissures. 



In his examination of the fossils of the coal-measures, Mr. Prest- 

 wich has shown that beds containing marine remains alternate with 

 others in which fresh-water shells and land plants occur, appearances 

 which he attributes to the flowing of a river, subject to occasional 

 freshes, into the sea, rather than to repeated changes in the relative 

 level of land and sea. 



It is certainly the safer course to incline to this hypothesis when- 

 ever there are no unequivocal signs, as in the Purbeck strata in 

 Portland, of land plants having become fossil on the very spots 

 where they grew. For although there may be many river deltas 

 like that of the Indus, where the land is subject to be alternately 

 upheaved above, and then let down below the waters of the sea, 

 yet such oscillations of level must be considered as exceptions to 

 the general condition of the earth's surface near the mouths of 

 rivers at any given period. Even in a case like the delta of the 

 Indus, both the causes above alluded to may be expected to co- 

 operate in producing alternate fluviatile and marine strata; for in 

 the long intervals between great movements of the land, the river 

 will annually advance upon the sea with its turbid waters, and then 

 retreat again as the periodical flood subsides, and the salt waters, 

 after being driven back for a time, will reoccupy the area from which 

 they have suffered a temporary expulsion. 



In the conclusion of his valuable paper, Mr. Prestwich observes 

 that the carboniferous strata of Coalbrook Dale must once have 

 been entirely concealed under a covering of new red sandstone, 

 and they owe their present exposure partly to those movements 



