The opposing view supposes that the gases were in the main original. Of this view there are two phases: (1) One supposes that the lavas are remnants of an original molten globe which absorbed gases from the primitive atmosphere and retained them till the time of their eruption. The possible absorption of steam and air into the supposed molten globe has been much neglected in current conceptions of early conditions. If the lavas of the supposed molten globe absorbed proportionately as much water-vapor as the volcanic lavas often contain, it would probably take forty or fifty times the present ocean and atmosphere to supply them. Any remnants of these original lavas might well be supposed to hold gases. Even rocks derived from them by deep-seated solidification might retain much gas. (2) The other phase of the view assumes that the gases were entrapped when the globe was built up of meteoroidal or planetesimal matter, as assumed in the accretion hypothesis.
Under either of the last two views the gases may be said to be primary, and genetically connected with the origin of the lavas themselves. Such gases would be a contribution to the atmosphere and the hydrosphere. This view does not exclude the idea that as the lava rises through the surface-rocks, other gases are formed by contact, and that they may be absorbed into the rising column. On the contrary, the view recognizes the possibility that a tongue of lava rising into the upper formations may encounter bodies of water, or masses of thoroughly water-soaked rock, from which great quantities of steam may be generated, and that this accessory steam may be a large factor in the initial explosion which often accompanies the development of a new volcano, or the new eruption of an old one after a long period of quiescence.
A decision on the vital question whether the volcanic gases are largely primary, or are essentially secondary, has not yet been reached; but it will doubtless be reached when a sufficient number of really representative analyses of volcanic gases have been made, and when the phenomena of the gases occluded in igneous rocks have been thoroughly investigated.
The peculiar proportions of the rock-gases, in which hydrogen and carbon dioxide so greatly preponderate, seem to imply that they are not derived from the atmosphere; at least if they were so derived, there must have been a selective absorption of a most remarkable kind, because hydrogen is present in the atmosphere in exceedingly small quantities, while carbon dioxide is a very minor constituent. At the same time, as already remarked, no free oxygen is usually found in these absorbed gases.
The question as to whether the larger part of the volcanic gases is original or is merely a special form of convective circulation, has an important bearing on the supply of the atmosphere, which is constantly being depleted by the oxidation of the rocks and by the formation of carbonates and carbonaceous deposits. This vital phase of the subject will receive further consideration. While recognizing the lack of decisive proof, it would seem that the preponderance of evidence lies in favor of the view that a notable portion, at least, of gaseous volcanic emanations is derived from the interior of the earth, and is really a contribution to the atmosphere and the hydrosphere. The hydrogen, on coming in contact with the atmosphere, ignites and adds itself to the hydrosphere. The carbon dioxide is in part decomposed by plants, and adds to the supply of atmospheric oxygen. The nitrogen, being comparatively inert, doubtless gradually accumulates in the air and has thus come to be its preponderant constituent.
THE CAUSE OF VULCANISM.
The extraordinary facts involved in volcanic phenomena cannot well be discussed fully until the origin of the earth is considered, and the great agencies, as well as the peculiar conditions, which the earth inherited from its birth, are duly weighed, for these were, with little doubt, the true causal antecedents of vulcanism. We shall return to the subject after a sketch of the early conditions of the earth, but the views that have been entertained may be reviewed here while the phenomena are fresh in mind.
The explanation of vulcanism involves two essential elements. These are (1) the origin of the lavas, which involves a consideration of the necessary temperatures, pressures, and other conditions, and (2) the forces by which the lavas are expelled.
Nearly all current explanations of vulcanism are founded upon conditions supposed to be derived from a molten globe, and fall under two general classes: (I) those which assume that the lavas are residual portions of the original molten mass, and (II) those which assign the lavas to the local melting of rock.