96 REPORT 1871. 



Oct. X7, it would also appear that the same agency was in operation at the 

 time of the previous sunset. 



1870, Nov. 1, 6'' to 6*" 40™. "A grand view again. Definition fair at 

 times. The margin of the eastern end of the floor very distinctly shaded, 

 showing that end to he convex as well as the western. This shading did not 

 conform to the general form of rim, but ran inwards (as shown in the 

 sketch) ; and three places on the floor were much brighter than the rest, 

 which was free from shading (their localities I have no doubt are those of 

 spots 3, 4, and 17), while the next bright parts of the floor are suggestive 

 of the light streaks; and the shading or lower part coinciding with the 

 narrowing of the streak between 4 and 3 as seen under higher illumination 

 in a measure supports the impression." 



The dip of the floor towards the border, as mentioned by Mr. Pratt, is 

 now well established by numerous observations, also the comparatively 

 greater elevation in the neighbourhood of the fault crossing Plato from 

 N.W. to S.E. These characteristics will probably afford some clue towards 

 framing a theory of the formation of the plain and rampart. Starting with 

 the now acknowledged principle that the moon manifests on a large scale the 

 operation of volcanic forces, we may first inquire as to their modus operandi 

 in the forms we observe. So far as we know, volcanos and earthquakes are 

 closely connected, and there is great reason to believe that both are the 

 results of expansion occasioned by the intumescence of material beneath the 



Pig. 8. 



crust or surface. It was, I believe, Scropc who first called attention to the 

 effect of the expansion of an iutumescent mass elevating the superincumbent 

 material ; and Hopkins, twenty-two years later, clearly showed that when 

 the surface was elevated to the point at which the tension and cohesion just 

 balanced each other, the slightest increase of tension ruptured the surface 

 and produced fissures, which might be considerably augmented by earthquake- 

 waves accompanied by the sudden subsidence of the tract between two 

 principal lines of fissures. In applying this reasoning to the explanation of 

 the formation of " Plato," the remarks of Scrope are so much to_ the point 

 that a transcription of them is essential to the due apprehension of the 

 forces concerned. 



In chapter x. of his ' Considerations of Yolcanos,' p. 205 (1825), Scrope, 

 speaking of M. de Buch's opinion that the intumescence and rise of the 

 basalt elevated the superincumbent strata, says : " 1 differ from him, inas- 

 much as I conceive the intumescence and rise of the basalt to be not 

 the cause but the result of the elevation of the overlying strata. 



"A general fact, noticed by M. de Buch himself, proves this most 

 thoroughly, viz. that wherever the basalt appears, the strata are invariably 

 found dipping toivards it, which is wholly inexplicable under the idea that 

 the basalt elevated them. ... If, however, we suppose the expansion of 

 the subterranean bed of crystalHne rock to have taken place at a great depth, 

 elevating the overlying strata irregularly along the line of various fissures, 



