EFFECTS OF EARTHQUAKES ON LAND. 151 



might perhaps be due to the burning of materials like 

 sulphur. Mr. D. Forbes, who examined the effects of the 

 earthquakes of Mendoza, which were felt for a distance of 

 1,200 miles, says that where the hard rock came to the 

 surface there were no traces of fissures, these being 

 entirely confined to the alluvium. The rumours of fire 

 and smoke having appeared at some of the fissures were 

 without foundation, the presumed smoke being nothing 

 but dust.^ 



In addition to flames lights appear often to have been 

 observed, the origin of which cannot be easily explained. 



The earthquake of November 22, 1751, at Grenoa 

 is said to have been accompanied by a light like that 

 of a prodigious fire which seemed to arise out of the 

 ground .2 



Explanation of fissure phenomena. — The manner in 

 which fissures are formed has already been explained 

 when referring to the want of support in the face of hills 

 (page 136). 



Similar remarks may be applied to the banks of rivers 

 and all depressions, whether natural or artificial, which 

 have a steep slope. At such places the wave of shock 

 emerges on a free surface, which, being unsupported in 

 the direction of its motion, tends to tear itself away 

 from the material behind, and form a fissure parallel to 

 the face of the free surface. The distance of the fissure 

 from the face of the free surface will, theoretically, be 

 equal to half the amplitude of the wave of motion, one 

 half tending to move forwards, and the other half back- 

 wards. The reason that water and other materials rush 

 forth from fissures has been explained by Schiiler as being 

 due to cracks having been opened through impervious 



» ProG. Geolog. Soe. Ap. 1875, p. 270. 

 ^ Getit. Mag. vol. xxi. p. 569. 

 8 



