TRANSACTIONS OF SECTION C. 669 



6. Note on the Occurrence of Levcite at Etna. 

 By H. J. Johnston-Lavis, M.B., F.G.S. 



Some years since, whilst on a visit to Etna, my attention was drawn to some 

 superficially placed tufis of a chocolate to a coffee-brown colour. In these tuffs, 

 near the Casa del Bosco, are observable included pieces of scoriaceous lava which 

 to the naked eye are evidently leucitic ; that mineral occurrinfj in large well-formed 

 crystals attaining to some millimetres in diameter, and brilliantly white as the 

 result of fairly advanced kaohnisatioii. In consequence of this change the rock is 

 excessively friable and, therefore, difficult to sectionise, A section of it, however, 

 was exhibited at the meeting and also two photo-micrographs therefrom. In these 

 it will be seen that kaolinisation has extended along the fracture planes of the 

 leucites, whilst the beautifully formed pyroxene crystals are unaltered and the 

 triclinic felspars are fairly in a normal condition. The base is a microlitic network 

 of felspar and pyroxene, together with beam if ul minute cubes and octahedra of 

 magnetite, rendering the substance intervening between the crystals almost opaque 

 even in thin sections. The pyroxene is often enveloped in a casing of leucite, as 

 at Vesuvius, Roccamonfina, &c., confirming what I have asserted in other places, 

 namely, that leucite is one of, if not the latest mineral to crystallise. 



I re-ret that I have not the opportunity of investigating the question of the 

 ori-^in and age of this rock more completely, as on writing to my friend Professor 

 {). Silvestri, inquiring if leucite had yet been encountered at Etna, I received a 

 categorical answer in the negative, which, coming from such an authority, must be 

 taken as conclusive as to the rarity of leucitic rocks being produced from Etna. 



The discovery of this mineral at Etna is what one would have looked for, 

 knowing as we do its wide distribution in nearly all the other late basic volcanoes 

 of Italy! 



7 Note on some recent Investigations into the Condition of the Interior of 

 the Earths By Professor E. W. Claypole, B.A., D.Sc, F.G.S. 



The difficulty of this great problem in geology was referred to in this paper as 

 a reason fur the slow progress made toward its complete solution, and the indirect 

 nature of the evidence was also quoted as a source of uncertainty. 



The chief element upon which reliance can be placed is the now seldom dis- 

 puted datum that the earth must in all these inquiries be regarded as a heated body 

 in cold space subject to laws of radiation as yet imperfectly understood. _ 



Reference was then made to the recent investigations of Mr. C. Davison, Mr. 

 T. M. Reade,Rev. A. Fisher, and Professor G. Darwin, claiming to prove that the 

 following deductions from this datum have been established :— 



(a) That below 400 miles the cooling, and consequently the contraction, are 



imperceptible. i i • • 



(6) That the cooling, and consequently the contraction, reach their maximum 



at the depth of seventy-two miles, 

 (c) That at the depth of five miles the contraction from cooling exactly equals 

 the diminution of space due to the descent of the shell at that depth 

 ensuing from the total vertical contraction of all the layers below it. 

 This layer at°the depth of five miles has, therefore, been termed ' the layer of 

 no strain,' being liable neither to extension nor compression, because the space is 

 exactly sufficient for its diminished bulk. 



The layer of ' no strain ' is placed by one of the authors above named at tbe 

 depth of five miles, and in a neutral zone between the bent and crushed strata 

 above it and the compressed and horizontally extended strata below it. It is ^con- 

 sequently impossible that any disturbance can occur in the layer of 'no strain. 



Yet in some parts of the earth, and notoriously in the Appalachian region ot 

 North America, strata have been forced up from a depth greatly exceeding this hmit. 



• Published in the American Geologht (Minneapolis, Minnesota) for June and 

 July 1888. 



