MR. ROBERT MALLET ON VOLCANIC ENERGY. 
171 
the whole machinery by which these huge heat-engines work ; and with the two 
together, all their ascertained phenomena as to ejecta and other products admit of easy 
explanation. 
We shall recur to these and compare the chief of these phenomena and of plutonic 
action generally with our cause for the local production of the heat itself, though 
necessarily briefly. For volcanic phenomena such as have always been known to us in 
historic time or by traces left before that, and which are characterized by a general 
uniformity of products and mode of action all over the globe, as well as in all time, 
differing only or mainly in intensity merely, the necessary coexistence of some source of 
high local temperature and of water to form steam is required. It is obvious, 
therefore, that no such volcanic action as we are now acquainted with can have existed 
on our globe prior to the deposition upon it of the great masses of superficial waters 
penetrating its solid materials by capillarity and infiltration to vast depths — that is to 
say, not until after the external surface of the globe had permanently fallen to the 
temperature at which liquid water could deposit and penetrate the earth’s crust ; and 
this fixes an anterior limit in time earlier than which vulcanicity, as we now know it, 
cannot have existed upon our globe. 
79. We have no very precise data for fixing this commencement of existing volcanic 
action in the geologic scale of succession, hut it probably does not go back much beyond 
the end of the Secondary period, if so far. Prior to that vulcanicity seems chiefly to have 
been developed in the welling up of huge volumes of liquid rock between severed 
masses, or masses of heated dust or so-called ash, and probably in other ways, but 
without ejecta due to elastic steam, though possibly to such occasionally due to gases, 
but in any case to have been different in its action from the present system, and to have 
formed but a part of the machinery of folding over and ridge elevation of the earlier 
stages of cooling and contraction. 
[It is not impossible that volcanic vents, or other sufficient evidences of true volcanic 
action of the explosive character now in play, may hereafter be discovered in the older 
sedimentary formations. The so-called deposits of “ volcanic ash,” the trap-dykes and 
porphyry-bosses of the Silurian rocks of the south of Ireland and of North Wales, &c., 
are evidences of igneous action indeed, but of that hydrostatic character which preceded 
the explosive volcanic action of the present epoch. 
Some of the phenomena of explosive action are occasionally observable in igneous 
formations undoubtedly not volcanic, as the greenstone or trap-boulders and pebbles 
found by the writer imbedded in the great greenstone or trap-dyke of Galway (Trans. 
Roy. Irish Acad., 1834) ; but no sufficient evidence exists, so far as his knowledge 
goes, of any volcano, properly so called, existing in Silurian times, nor for long after. 
Nor, if the existence sporadically of such were proved, would that controvert the 
writer’s view, that the great system of explosive volcanic vents as now established on 
our globe does not date back in the main further than above stated. No precise 
boundary in time can be assigned for the passage from the hydrostatic to the explosive 
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