382 Correspondence — Dr. H. J. Johnston-Lavis. 



many facts marshalled by him in his discussion of Professor Shand's 

 paper in the Geological Magazine.^ 



It is for nearly thirty years now that I have been preaching 

 the same sermon — the * cooling history ' or, as I prefer to call it, 

 the 'vicissitudes of consolidation', which I may sum up under the 

 following heads : — 



1. Primary composition of the paste. 



2. Additions, or rather acquisitions, to it in its progress from its 

 source to its final position : (a) assimilation of materials from its 

 enclosing rock, (b) materials brought to it with the assimilation of 

 saline solutions, gases, etc., otherwise endosinotic raetamorphism. 



3. Losses from it in its progress from its source to its final 

 position : {a) abandonment of materials to its enclosing rock, {b) 

 losses of soluble materials to fluids circulating in its vicinity, or losses 

 of volatile constituents escaping by pores, fissures, or a chimney 

 opening to the atmosphere, etc., otherwise exosmotic metamorphism. 



4. Rate of cooling or interruptions of cooling : («) to surrounding- 

 rocks, {b) loss in the acquisition of Hg or other materials, volatile or 

 otherwise, {c) conduction by surrounding rocks, {d) evolution of 

 volatile materials, (e) vesiculation, or the conversion of volatile 

 materials from the volume of a liquid dissolved in a liquid to th& 

 state of a gas, (/) ebullition and sublimation of volatile materials by 

 fumaroles or an open volcanic vent. 



5. Diminution, rapid or slow, and other vicissitudes of pressure. 



I was able to show that, of the sister minerals, orthoclase was 

 formed under high pressure, whilst leucite was individualized under 

 low pressure and principally at an open volcanic vent, as exhibited 

 by the large crystals of leucite growing over big plates of sanidine. 

 The same was shown with regard to amphibole and augite. 

 Furthermore, the presence of small quantities of some bodies that 

 have been called mineralizers, but may well be called catalyzers, may 

 be added to my list of ' vicissitudes of consolidation '. 



What determines, for instance, in low-pressure minerals, the final 

 separation of the three felspathoids, leucite, haiiyne, or nosean, all 

 of which appear in almost identical conditions in the lavas of 

 Monte Yultura and other localities ? Why do abyssal minerals, such 

 as amphibole, sommite, biotite, and, I can lately add, garnet, be 

 deposited under practically no pressure in the loose pipernoid tuffs 

 of the Campania? I have specimens of bones of Cervus elaphus 

 (which I have described) covered with crystals of amphibole and 

 a nepheline mineral (sommite or micro-sommite) and biotite, which 

 must have been deposited at so low a temperature that even now the 

 gelatinous constituent of these bones has not been carbonized, but, by 

 heating over a flame, can be blackened yet covered, as such specimens 



pp. 302-7, 1885. "The Kelationship of the Structure of Igneous Eocks to 

 the Conditions of their Formation " : Sci. Proc. E. Dublin Soc, N.S., vol. v, 

 pp. 112-56, 1886; see also Q.J.G.S., vol. xli, pp. 103-6. "On the Fragmentary 

 Ejecta of Volcanoes " : Proc. Geol. Assoc, vol. ix, pp. 421-32. " The Causes 

 of Variation in the Composition of Igneous Eocks " : Nat. Sci., vol. iv, 

 pp. 134-40, 1894. 



^ See paper by Professor S. J. Shand in Geol. Mag. for 1913, pp. 508-14. 



