DIKES PREVIOUSLY DESCRIBED 259 



Other dikes in the region of the Mendips hills are described in more 

 or less detail by Mr Moore. Regarding the origin of these remarkable 

 dikes Mr Moore has the following to say : * 



" Under certain circumstances they (the Liassic limestones) are found either rest- 

 ing immediately upon the limestone {Carboniferous) ,1- fiWin^ up any basins or irreg- 

 ularities in its surface (when there have been any opportunities), passing down into 

 its fissures, or lying against the southern outer edge of what there is every reason 

 to suppose was the ancient Rhaetic and Liassic coastline presented by the Mendips 

 at these and subsequent periods." 



Further, in remarking on the origin of the dikes near Holwell, the 

 following is said : X 



" The curious phenomena here observed, and especially the thickening of the 

 dikes of more recent age downwards, are somewhat difficult to account for. An 

 analogous state of things might arise, could we suppose that the Carboniferous 

 limestone at this point formed the wave-washed clifi" of the Liassic sea, and that 

 its caverns were subsequently filled by Liassic deposits. A longitudinal section at 

 their junction would then show similar conditions ; but we cannot strictly adopt 

 this idea, from the fact that the same phenomena prevail throughout the whole 

 line of the Carboniferous limestone, and that the veins are continued to unascer- 

 tained depths." 



Regarding the age of the dikes described by Mr Moore, Professor 

 Woodward says : § 



'* It is possible that there were open fissures on the sea-coast in Liassic times, 

 as we see at the present day in the Carboniferous limestone near Sutton in South 

 Wales, but the admixture of fossils suggests that in-fillings may have taken place 

 at various periods, in some cases perhaps subsequent to the Jurassic epoch. In 

 some of their features these Liassic veins resemble chasms and * pipes ' in the 

 Kentish Rag of Maidstone, where in-fillings of fossiliferous brick earth occur." 



Oldham and Mallet, 1872. In 1872 Messrs Oldham and Mallet men- 

 tioned the formation of fissures, and the injection of sand into them, in 

 Cachar, India, by the earthquake of 1869. || 



During the earthquake many fissures were formed over a wide area in 

 "layers of stiff clay and mere sandy deposits some 25 or 30 feet thick, 

 which, throughout, rest upon a bed of some 3, 5, or 6 feet thick of bluish 

 silt or sandy ooze . . .'' 



The underlying watery " silt or sandy ooze " was forced up into the 

 fissures under such pressure in some cases as to cause it to spout up above 



*Ibid., p. 482. 



t Italics in parentheses are the present writer's. 



J Ibid., p. 482. 



§ Horace B. Woodward : The Jurassic Rocks of Great Britain. The Lias of England and Wales. 

 (Yorkshire excepted.) Memoirs of the Geological Survey of the United Kingdom, vol. iii, p. 98. 



II Notice of some of the secondary effects of the earthquake of 10th January, 18G9, in Cachar. 

 Communicated by Doctor Oldham. With remarks by Robert Mallet. Quart. Jour, of the Geol. 

 Soc. of London, vol. 28, 1872, pp. 2.55-270. 



XXXVII— Buu,. Geoi,. Soc. Am., Vol. 14, 1902 



