174 



Dt'uer data : the height of the tides and the amounts of precession and nutation 

 demand in the earth as a whole, a rigidity greater than steel, and the fact of 

 the comparative softness of the upper crust with whicli we are familiar, 

 compels us to infer that no less thickness than half the radius would admit 

 of these phenomena remaining as they are. Mr. Ansted, in his recent Rede 

 lecture, regards these calculations of great iuiportauce to physical geology, 

 and considers it probable that we shall have to explain the various 

 phenomena of volcanoes and earthquakes as being so superficial as to have 

 no reference to any fluid contents of the earth's interior. Sir C. Lyell promises 

 some further information on this subject in the forthcoming second volume of 

 the "Principles ": it is not unlikely that he will accept Mr. Hopkins' admission 

 of the possible existence vjithin the earth's crust of vast lakes or seas of lava. 

 With reference also to the third question, the method of volcanic action, our 

 knowledge is stiU very imperfect'; we believe that the earth has within it some 

 certain centre or centres of heat, that tinder different conditions this heat 

 manifests itself in different ways, that the causation though locally variable 

 has been identical throughout the ages, in the lavas which roasted the black 

 shales of Fowlet's Farm, the eruptions of Vesuvius, the Hereford eartliquake, 

 and the calamities of the "West India Islands. We cannot penetrate into the 

 secrets of Nature's subterranean laboratory : we shall probably in time become 

 more familiar vidth her mysterious procedure, and phenomena which now appear 

 paroxysmal will be reduced into order and be proved to have a cyclical relation. 



At Woolhope the volcanic action has displayed itself by a continuous 

 effort on the part of the Igneous rock to thrust itself forth from the earth ; the 

 heat has been converted into a mechanical force, which has broken the continuity 

 of the overlying beds and severed them wide asunder; that the Upper Llan- 

 dovery rocks have not been torn apart may be explained on the supposition that 

 they were formerly more pliant than they are now ; such beds, according to LyoU 

 (Elements p. 58), may have owed their flexibility, partly to the fluid matter 

 which they contained in their minute pores, and partly to the permeation of 

 sea-water while they were yet submerged. A glance at Fig. 2 will show the 

 strain to which they have been subjected. 



In Fig. 1 a rough attempt has been made to explain generally the 

 variation of dips visible in a valley of elevation. In every published section 

 of our district I endeavoured, without success, to describe a circle to which 

 the external ridges might be tangents ; this would have been possible if the 

 upheaval had been as in the dotted lines ; I found, however, that only by 

 producing the eastern ridge was it possible to describe such a circle, and I 

 inferred that the axis of the force must have been perpendicular to the chord 

 joining the points of contact A B. Or reversing the steps of the reasoning, 

 it is evident that when the axis of elevation is vertical, as / c, the external 

 beds will be found dipping everywhere at equal angles from the centre as 

 c a b—c h a. If, however, the axis is inclined as F C, though the force will 



