264 Russell — Massive-Solid Volcanic Eruptions. 



without conspicuous crystals, and (3) irregularly jointed but 

 without a well denned columnar-structure. Each of these 

 propositions may for convenience be considered separately : 



1. A compact rather than a scoriaceous condition of the lava 

 forced out during massive-solid eruptions is to be expected 

 from the fact that preceding liquid or fragmental-solid dis- 

 charges would have removed the more thoroughly vapor- 

 charged summit portion of the rising column, while the 

 material at a greater depth, less thoroughly vapor-charged and 

 congealing under pressure, would form a compact lava. Turn- 

 ing to the known examples of massive-solid eruptions available 

 for comparison, we find this conclusion sustained in the case of 

 the central crags in the Oregon crater described above, which 

 consists of compact granular material. 



2. A stony or granular texture, without well defined or 

 conspicuous crystals, would be expected because the lava consoli- 

 dates with comparative rapidity near the summit of the con- 

 duit from which it is later extruded, thus not allowing suffi- 

 cient time for an advanced stage of crystallization. On the other 

 hand, cooling takes place less rapidly than in the surface por- 

 tion of a lava sheet, and a glassy texture would not be expected. 

 The rock in question should seemingly be intermediate in struc- 

 ture between those which cool slowly, as in intruded sheets 

 and the central parts of thick lava flows, and those which cool 

 so rapidly that a glass (obsidian) results. Although this rea- 

 soning seems to be logical, yet, as is well known, crystals are 

 sometimes formed in deep-seated magmas and are carried to the 

 surface when the containing magmas migrate outward and are 

 discharged by volcanoes. This phase of the problem is 

 obscure, and possibly a granular or crypto-crystalline structure 

 may not be an essential characteristic of lava extruded in a 

 massive-solid condition. 



Turning again to the known examples, we find the rock com- 

 posing the crags in Panum crater, and the one in Oregon 

 described above, to be stony in texture, and in the case of the 

 Oregon crater at least, without either porphyritic crystals on 

 the one hand, or obsidian or pumice on the other. 



3. Columnar structure in igneous rocks, as is well known, 

 results from slow cooling, and the columns formed under such 

 conditions have their longer axes at right angles to the cooling 

 surfaces. As the material forced out during massive-solid erup- 

 tions is still hot when it reaches the air, and as the diameter of 

 such extruded masses, so far as known, is but a few hundred feet, 

 it is to be expected that cooling would progress too rapidly and 

 too irregularly to permit of the formation of systematically 

 arranged joints, and hence, a well-defined columnar structure 

 would be absent. If in large extrusions the rate of cooling 

 did permit of the origin of a columnar structure, the columns 



