THE BURNING CLIFF AT LYME REGIS. 159 



on a gentle slope will gradually move down it in consequence of 

 alternating contraction and expansion caused by differences of 

 temperature. 



There must come a time when the tension is strong enough to 

 overcome the cohesion of some fissured mass to the rock at the 

 back of it, and then a fall of cliff or a landslip takes place. The 

 movement of one slip is likely to loosen other unstable masses, 

 and even the vibration caused by a slip or fall is enough to start 

 adjoining masses on a downward journey ; so that one fall is often 

 followed, sooner or later, by other falls. 



It is possible, moreover, that in some cases a terrestrial tremor 

 or small seismic wave may be the final and immediate cause of 

 the severance of cohesion. Such earth-tremors are of very 

 frequent occurrence, but most of them are ot such small intensity 

 that they are only perceived by the sensitive instrumentality of a 

 seismograph. There is, however, ever}' gradation between such 

 feeble tremors and one which might be called a slight shock of 

 earthquake. As the cliff-fall at Lyme seemed to have happened 

 very suddenly and without any warning symptoms, the possibility 

 of its having been started by an earth-tremor occurred to Mr. 

 Cameron, and on his suggestion, I wrote to Mr. J. Milne. He 

 informs me, however, that his instruments did not record any 

 earthquake movement on the xoth of June, and consequently 

 the precise moment of the landslip cannot have been determined 

 in this way. 



It is important to remember that this landslip took place 

 during dry weather. April had been a wet month, but the latter 

 part of May and the beginning of June were dry and sunny, so 

 that I think we may infer that the final cause of detachment was 

 contraction, owing to evaporation of moisture and drying of the 

 material which forms the cliffs. 



It should also be pointed out that the geological conditions on 

 the east side of Lyme Regis are very different from those 

 existing between Lyme and Axmouth, where masses of Greensand 

 and Chalk rest on Lias clays or Triassic marls. There the base 

 of the Greensand descends to a lower level, so that there is a 



