Dr. H. J. Johnston-Lads — On Volcanic Action. 247 



governing eruptive action." This paper, for some unknown reason, 

 was suppressed; but in 1886 it was at last published in the "Pro- 

 ceedings of the Royal Dublin Society." That memoir, though so 

 much delayed, has precedence of the interesting studies of Lagorio, 

 and recent]}^ of Iddings. 



Although these preliminary statements might appear as outside 

 the present subject, they are not so ; for it is necessary to separate 

 absolutely the superficial from the deep phenomena of the eruption 

 of igneous matter, and reference to those papers should be made, so 

 that it may be seen that there such distinction has been made, and 

 a full explanation of the superficial ones given. What was assumed 

 as necessary to that explanation was, if required, an unlimited amount 

 of igneous matter coming from below. It is just this assumption 

 that is now to be discussed in the light of the new theory of secular 

 straining of the earth which I contend will give the first clear 

 and satisfactory explanation of deep volcanic action. 



In the old theory of the earth-crust crumpling over a contracting 

 and cooling nucleus, fluid or partially so, it always appeared to me 

 to be inexplicable how fluid matter could be squeezed out, or why 

 all the fluid on the surface of the earth did not rush down to fill up 

 any vacancy that the contracting interior tended to produce. 



This perhaps is expressing the facts in simple commonplace 

 terms, but is sufficient to illustrate the incompatibility of this 

 hypothesis with the fact of some of the liquid interior of the earth 

 rising through fissures towards the surface. 



The hypothesis that tangential thrust did not exist, but that the 

 earth-crust was shrinking and compressing an entire or partial fluid 

 nucleus, would have satisfied the vulcanologist, but is absurdly con- 

 trary to the incontrovertible evidence of tangential compression, as 

 seen in the plications and overthrusts existing upon the whole 

 surface of the globe. 



The Mellard Reade and Davison theory is reasonable in itself, and 

 satisfies alike the student of mountain building and the vulcanologist. 

 Prof. 0. Fisher's criticisms at first sight seem rather serious to the 

 theory itself; but when we consider the most flimsy nature of the 

 data of the age of our earth, the temperature of solidification and 

 the rate of cooling, we may safely say that the mathematician has 

 yet to have his day. Besides this, even the nature, and still more 

 the conductivity of all rocks beyond two miles from the surface 

 is unknown. 



The new theory tells us that the shells from the uncooled nucleus 

 out to the zone of maximum cooling lose more and more heat; and 

 their tangential strain increases— a condition in fact identical with 

 the guns built up of steel rings, shrunk on, one above another. 

 The tendency of all the shells beneath the no-strain zone is not only 

 to crumple all above it, but to compress the uncooling nucleus ; but 

 when that compression has reached its maximum limit, equilibrium 

 can only be restored either by fluxion stretching or shearing (how 

 about foliation) or fracture. This fracturing would tend to be 

 fusiform in section (I am open to correction) with the maximum 



