Propositions Concerning the Classification of Lavas, Considered with 

 Reference to the Circumstance of their Extrusions. 



By N. S. Shaler. 



1 HE lavas of the earth are not easily divided into distinct physical groups ; a division on 

 ^he basis of their chemical history has not yet been satisfactorily made, even Richtofen's 

 ingenious classification, which at first was accepted with considerable confidence, and seemed 

 to promise a basis for future enquiry, not appearing to satisfy the conditions in all 

 the various regions where volcanic rocks of different ages are found. The question of 

 the physical and chemical history of the various molten rocks is still so much open to 

 debate that almost any suggestions which promise to aid our understanding may be 

 deemed warranted. I therefore venture to present the following considerations concerning 

 the physical history of the known groups of lavas. In this paper I propose to limit my- 

 self to the mechanical history of the formation and the extrusion of lavas, avoiding the 

 field of chemical inquiries as far as it may be possible to do so. For convenience, I have 

 arranged the following matter in the form of propositions which are taken up in succession. 

 I regret the appearance of dogmatic assertion this form gives, and also regret that the 

 limits of the paper, as well as my knowledge of the work of others, do not enable me to 

 indicate the precise points in which the suggestions differ from those which have 

 hitherto been made. 



Outflows of lava are pretty clearly divisible into three classes, viz : 



1. Those of ordinary volcanos, characterized by the fact that the escape of gas under 

 great pressure brings about the extrusion of the lava. 



2. Massive outflows of lava flowing not from a cone, but from a fissure, and showing 

 no trace of the gas action which is the most -conspicuous feature in true volcanic action. 



3. Inclosed dykes, i. e., dykes whose fissures did not reach the surface, but stopped 

 short of it within the rocks which they penetrate. 



Besides these we have in various dykes and volcanic chimneys, occasional relics of the 

 old channels up which the lavas that were forced out in volcanic or marine eruptions found 

 their way to the surface. It is often impossible to determine to which of these last 

 named classes such ejections' belong, but they are easily separable from the third group, 

 i. e., that of enclosed dykes. 



