28 E. W. Wetherell — Xanthidia in the London Clay. 



It is a question, however, whether the plications are wholly 

 formed by expansive compression ; if they are, then they become 

 a measure of the expansion. Now in the case of the Alps the cal- 

 culated shortening is nearly as great as the actual width of the 

 range ; in other words, the expansion must have doubled the length 

 of the beds laterally ; to do this it must have doubled the volume 

 of the solid mass out of which the range was formed. Let us apply 

 this result to an area of the size used by Mr. Eeade for illustrative 

 calculation, viz. one measuring 500x500x20 miles, which is equal 

 to 500,000 cubic miles. Now if this is doubled by expansion, 

 600,000 cubic miles has been added to the mass, but the cal- 

 culated expansion of such a block raised by 1000° F. is only 78,400 

 cubic miles, and is therefore very far short of the required amount. 

 The conclusion I would draw from this little calculation is that the 

 plications are not wholly due to expansion. 



Finally, it seems desirable to point out to Mr. Eeade that the 

 assumption of a plastic substratum will not satisf}'^ Sir W. Thomson's 

 demands for the rigidity of the Earth as a whole. Mr. Reade can- 

 not have a substratum that is plastic enough to yield to such a small 

 terrestrial influence as the local accumulation of sediment, while it 

 is rigid enough to resist the deforming tidal influences of the sun and 

 moon. The only way out of this difficulty is the assumption of 

 a liquid substratum saturated with dissolved gas : this is Mr. 

 Fisher's view, and it affords a much more satisfactory basis for the 

 explanation of terrestrial movements. 



Postscript, — Correspondence with Mr. Davison leads me to see 

 that although the temperature of the mass below the depressed area 

 will be raised by conduction of heat from the surrounding parts, yet 

 that these surrounding parts will contract in parting with heat ; 

 consequently if this conduction is fairly rapid and the further equali- 

 zation of temperature is a slow process, the mass which is receiving 

 heat may expand laterally into the spaces formed by the contraction 

 of the parts immediately outside it, instead of expanding upwards 

 and raising the depressed crust. Mr. Davison therefore still main- 

 tains that no upheaval would result from the expansion of a 

 depressed portion of the crust, and until his argument can be 

 answered it certainly cuts at the root of the expansion theory of 

 surface upheavals. 



V. — On the Occdrrenoe of Xanthidia (Spixiferites of Mantell) 



IN THE London Clay of the Isle of Sheppey. 



By E. W. Wetherell, F.G.S. 



THE vexed question of the affinities of the fossil organisms known 

 as Xanthidia — a question which only experienced zoologists 

 and botanists can hope to solve — will not be discussed in this short 

 paper, my intention being merely to show that these interesting 

 organisms are not confined (in England) to the Chalk flints and 

 Grey Chalk, but exist in the London Clay, and to give a short account 



