38 J. Clifton Ward — Modem Vulcanicity. 



EXPLANATION OF PLATE II. 



Fig. la. — Heterodictya gigantea. A broken frond, of the natural size. The specimen 



is split longitudinally along the line of the central laminar axis, and thus shows 



the bases of the cells. 

 Fig. lb. — Portion of the same, near its smaller end, enlarged to show the penniform 



arrangement of the cells. 

 Fig. Ic. — Transverse section of the frond, of the natural size. 

 Fig. Id. — A few of the cells of the same viewed in profile, showing the tabulse. 



Enlarged. 

 Fig. \e. — A small portion of the surface greatly enlarged, showing the shape of the 



bases of the cells. On the left hand side of the figure, a portion of the longitu- 

 - dinally striated laminar axis is preserved. 

 Fig. 2. — Ptilodictya cosciniformis, a broken specimen growing on Heliophyllum Halli. 



Of the natural size. 

 Fig. 2a. — Portion of the same enlarged, showing the meshes of the network, and 



their striated borders. 

 Fig. 2b. — A portion of the same still further enlarged, showing the form and arrange- 

 ment of the cells and the interstitial tubuli. 

 Fig. 3. — Fenestella Davidsoni ; a small portion of the non-poriferous side. Of the 



natural size. 

 Fig. 3a. — Portion of the same enlarged. 



Fig. 3b. — Portion of the poriferous side of another specimen of the same, enlarged. 

 Fig. 3c. — Small portion of a branch of another example of the same, greatly enlarged. 

 Fig. 4. — Fragment of Retepora Trentonensis, of the natural size. 

 Fig. la. — Portion of the same enlarged, showing the arrangement of the cells. 

 Fig. 45. — Portion of another example of the same, enlarged; showing the striated 



non-poriferous surface. 

 Fig. 5. — A small crust of Ceramopora Huronesis, growing on Heliophyllum Halli, 



enlarged. 

 Fig. 5a.— Portion of the same, greatly enlarged, showing the form of the cells and 



cell-mouths. 



Y. — Modern 'Vulcanicity.' 1 



By J. Clifton Ward, 



Assoc. R.S.M., F.G.S., of H.M. Geological Survey. 



THE theory proposed of late years by Mr. Mallet to account for 

 volcanic and earthquake phenomena, while having a charm 

 about it from its very simplicity, is one which must nevertheless 

 meet with very decided criticism from geologists. 



Mr. Mallet sees " linked together, as parts of one grand play of 

 forces," the elevation of mountain-chains, the production of volcanos, 

 and the origination of earthquakes. His theory, however, necessitates 

 the following suppositions : — 



1. '" That the geological doctrine of absolute uniformity cannot be 

 true as to Vulcanicity. ... Its development was greatest at its 

 earliest stages." (p. 75.) ' 



2. That the movements of elevation and depression at the present 

 time are "slow and small," but these, "at a much remoter epoch, 

 acted upon a much grander and more effective scale." (p. 62. ) l 



On page 47 'the " stratigraphic geologist" is described as one who 

 discerns "a change in the order or character" of the "fused masses 

 which have come up from beneath. He sees immense outpourings of 

 granitoid or porphyritic rocks that have welled up and overflowed 

 the oldest strata. . . . Later he sees huge tables of basaltic rock 



1 Introductory Sketch to Palmieri's Vesuvius. 



