The Monteregian Hills. 243 



shown in Fig. 7. The opening occupied by the intrusion 

 was in all probability formed by the perforation of the 

 horizontal shales at this point by the explosive action of 

 the steam and vapors preceding the eruption proper, as it 

 presents exactly the features reproduced by Daubr^e in 

 his highly suggestive experiments on the penetrating 

 action of exploding gases. It is, in fact, what he terms a 

 diatreme. 



Des perforations aussi remarkables, tant par leurs formes que par les 

 communications qu'elles ont etablies avec les profondeurs du sol, con- 

 stituent, parmi les cassures terrestres, un type assez nettement 

 characterise pour meriter d'etre distingue par une denomination pre- 

 cise et cosmopolite. Le nom de diatreme rapelle l'origine probable de 

 ces trouees naturelles, veritables tunnels verticavx, qui se rattachent 

 souvent, comme mi incident particulier, aux cassures lineaires, dia- 

 clases et paraclases. 1 



Fig. 7- — Diagrammatic cross-section of Mount Johnson, showing 

 the relation of the several rock types. 



The occurrence is one which presents a close re- 

 semblance to the remarkable volcanic necks recently 

 described by Sir Archibald Geikie 2 in East Fife, and also 

 to those described by Branco, 3 in Wtirtemberg. Mount 

 Johnson, however, is a neck occurring in an area which 

 has undergone much more extensive denudation since the 

 time of the intrusion than in the cases above mentioned, 

 and as a consequence of this the fragmental material 



1 "Recherches experimentales sur le role possible des gaz a hautes 

 temperatures doues de tres fortes pressions, etc!," Bull, de laSoc. Gdol. 

 de France, 3e serie, tome XIX (1891), p. 328. 



2 The Volcanic Necks of East Fife. Glasgow : Hedderwich & Sons. 



3 Schwabens 125 Vtdcan-Embryonen und deren tufferfiillte Ausbrurhs- 

 rohren das grosste Gebiet ehemaliger Maare avf der Erde. Tubingen , 

 1894. 



