252 Dr. Johnston- Lavis — Eruptive Rocks of Gran, Norway. 



III. — The Basic Eruptive Eooks of Gran (Norway) and their 



Interpretation. A Criticism. 



By H. J. Johnston-Lavis, M.D., F.G.S., etc. ; 



Prof, (pareg.) of Vulcanolog-y in tlie R. Univ. of Naples. 



BEING present at the reading of the abstract of my friend Prof. 

 Brogger's paper at the last meeting of the British Association, 

 and having listened to certain laudations thereon, I, with several 

 others, went away with the profound conviction that our Scandinavian 

 colleague was about to bring down upon us a mass of evidence 

 bearing on the differentiation of rocks as irresistible and over- 

 whelming as that of his Viking ancestors in times gone by. Since 

 then, however, calmly reading over his admirable memoir recently 

 published in the Quart. Journ. Geol. Soc.,' what was my astonish- 

 ment to find that this veritable avalanche of facts brought forward 

 to support a preconceived hypothesis is of inestimable value as 

 material evidence to be used against the theory of internal chemical 

 segregation, as maintained by that author. 



We have described to us several comparatively small eruptive 

 plugs of varying shape the material of which has forced its way 

 through Silurian and Devonian shales and limestones; and consider- 

 ing their limited dimensions has produced intense and widespread 

 contact metamoi'phism in the country rocks around them by the 

 formation of minerals which Prof. Brogger admits in some cases 

 to have derived part of their constituents from the metamorphosing 

 magma ; yet my friend asks us to believe that all the varieties of 

 composition in the igneous rocks are due to chemical segregation. 

 We are told likewise that as we examine the eruptives from north 

 to south, we find them become less and less basic, and this is given 

 as evidence of segregation in the magma taking place in horizontal 

 extension ; yet nothing is said of the variations in the composition 

 of the country rock in the same direction. 



In the next place the nuclear part of these eruptive stumps is 

 shown to be less basic than the peripheral. This is just what we 

 should expect, and corresponds with the arrangement as displayed 

 in many other dissections of ancient volcanic districts. Prof. Brogger 

 would have us believe that this shows the segregation of a more 

 basic paste near the cooler walls of the cavity, and the concentration 

 of the acid portion in the central and hotter part of the reservoir. 

 Now these facts can be interpreted in quite a different manner. 

 Have we not here strong evidence of the osmotic theory, which I 

 maintain is the true general explanation of the variation in eruptive 

 rock composition. A small conduit of an unknown original chemical 

 constituent has, in passing through basic sedimentary rocks, produced 

 in these an intense physico-chemical change and extensive develop- 

 ment of new minerals at the expense of the loss of silica, alumina, 

 alkalies and probably other constituents, whilst a very marked gain 

 has taken place with regard to the amount of magnesia and lime. 

 Without an elaborate series of analyses of the alteration products 

 in the surrounding rocks, we can only form a most inaccurate idea 



1 Vol. L. part 1, February, 1894, pp. 15-37. 



