246 Dr. LT. J. Johnston- Lavis — On Volcanic Action. 



Tliis species resembles Thracia Wilsoni, Moore (vol. cit. p. 254, 

 pi. xiv. fig. 8, figured as Corimya Wilsoni, Moore, in Quensl. Pal. 

 pi. 28, figs. 10, 11). It is chiefly distinguished from that shell by the 

 difference between the anterior and posterior breadth, and possibly 

 also by its smaller habit of growth. 



Locality. — Primrose Springs, north of Lake Eyre. 



EXPLANATION OF PLATE IX. 



South Australian Fossils. 



Fig. 1. Ammonites foniinalis, sp.n. Primrose Springs, Lake Eyre. 



,, 2. Alaria (? Anchura) sp. cf. Anchura carinata, Mantell. Primrose Springs. 



,, 3. ? Turbo, sp., cf. Turbo Icevigatus, Sow. Primrose Springs. 



,, 4. Actceon or Avellana, species. ? Primrose Springs. 



,, 5. Pecten, species. Mt. Hamilton Station, near Lake Eyre. 



,, 6. Pinna auntralis, sp.n. Sandy bed at Primrose Springs. 



,, 7. Thracia primula, sp.n. (right valve). Primrose Springs. 



,, 8. Modiola subsolenoides, sp.n. No locality. 



,, 9. Mytilus, sp. No locality. 



II. — The Extension of the Mellard Keade and C. Davison 

 Theory of Secular Straining of the Earth to the Explana- 

 tion of the Deep Phenomena of Volcanic Action. 



By H. J. Johnston-Lavis, M.D., B.-es-Sc, F.G.S., etc. 



FOR many years my thoughts have been occupied with the 

 phenomena of eruption at or near the surface, and although 

 a detailed study of the Vesuvian volcano and its products gave me 

 the key to the mechanism of eruption and injection of igneous 

 matter near the surface, I was always at a loss to understand, on 

 the older theories, how lava could ever start upwards in fissures. 



It has been said that volcanic action was the cardinal point upon 

 which hung the hypotheses and theories which have been or may be 

 offered in explanation of those geodynamical problems which are 

 the very basis of geology. Six years since, that group of phenomena 

 which constitute eruptive activity at the surface of our planet were 

 recognized as due to the presence of elastic vapours contained 

 within the fused rock ; but no ideas had been offered, and still less 

 any law formulated, of the introduction into and separation of 

 these volatile constituents from the igneous magma which is the 

 cause of all the modifications of eruptive actions as we see it. In 

 my paper on the Geology of Monte Somma and Vesuvius, the con- 

 clusions I had been led to by the study of that volcano, as illustrating 

 surface eruptive phenomena in general, were given and the paper 

 was terminated by 50 propositions which appeared to me as being 

 supported by those studies. 1 After my further investigations at 

 other volcanoes, finding an overpowering abundance of confirmatory 

 evidence, those conclusions were united in another paper of mine 

 read before the Geological Society on April 29th, 1885, entitled, 

 " The Physical Conditions involved in the Injection, Extrusion, and 

 Cooling of Igneous Matter," and which enunciate the physical laws 



1 These conclusions and propositions, contrary to my wishes, were not allowed to 

 appear in the memoir at the time it was published, Q.J.G.S. Feb. 1884. 



