“ih 
The evidence of this is to be found in the direction 
of the banding or fluidal arrangement of the crystals in 
the essexite already referred to and shown in the accom- 
panying illustration. This fluidal arrangement is seen in 
most large exposures of the essexite and with especial 
distinctness in the great faces of this rock exposed in the 
quarries on the mountain side, and it is always vertical, 
showing that the movement of the rock was upward through 
the pipe, and not outward and horizontally over the pulas- 
kite, as it would have been in the case of a laccolite. Fur- 
thermore, in several cases when the fluidal arrangement 
is very distinct and has a somewhat banded character 
due to the alternation of somewhat more feldspathic 
portions of the rock with others richer in iron-magnesia 
constituents, a strike can be made out on horizontal sur- 
faces, and this strike curves around the mountain, following 
its marginal outline, as shown on the map. 
It is thus clear that Mount Johnson is a neck in its 
most typical form. A cross-section of the mountain is 
shown in the accompanying figure. 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 vapours preceding the 
eruption proper, as it presents exactly the features repro- 
duced by Daubrée in his highly suggestive experiments on 
the penetrating action of exploding gases. It is, in fact 
what he terms a diatréme. 
“ Des perforations aussi remarkables, tant par leurs formes que par 
les communications qu’elles ont établies avec les profondeurs du sol, 
constituent, parmi les cassures terrestres, un type assez nettement 
charactérisé pour mériter d’étre distingué par une dénomination 
précise et cosmopolite. Le nom de diatréme rapelle l’origine probable 
de ces trouées naturelles, véritables tunnels verticaux, qui se rattachent 
souvent, comme un incident particulier, aux cassures linéaires, 
diaclases et paraclases.*”’ 
The occurrence is one which presents a close resem- 
blance to the remarkable volcanic necks described by Sir 
Archibald Geikie** in East Fife, and also to those described 
by Brancof, in Wurtemberg. Mount Johnson, however, 
is a neck occurring in an area which has undergone much 
*** Recherches expérimentales sur le réle possible des gaz a hautes températures 
doués de trés fortes pressions, etc.,’’ Bul. de la Soc. Géol. de France, 3e série, tome 
XIX. (1891) p. 238. 
**The Vocanic Necks of East Fife. Glasgow: Hedderwich & Sons. 
tSchwabens 125 Vulcan-Embryonen und deren tufferfiillte Ausbrurhsréhren das 
grosste Gebiet ehemaliger Maare auf der Erde. Tiibingen, 1894. 
