1856.] SCROPE — CRATERS AND LAVAS. 32/ 



sufficiently accounts for the different reception these two works met 

 with from geologists at the time. Neither, however, I presume to 

 hope, were wholly without some beneficial result. At the period of 

 their publication, the Wernerian theory of the precipitation from 

 some aqueous menstruum, not merely of granite, and what were then 

 called the primitive formations, but even of all the trap-rocks, still 

 prevailed, and had the support of a large school of geologists in this 

 country. I venture to think that the facts reported in my two 

 volumes (especially those represented to the eye in the atlas illustra- 

 tive of the volcanic remains of Central France) had some share in 

 the final extinction of that German romance, — which some geologists 

 as old as myself may remember to have been regarded almost in the 

 light of a gospel-truth, and defended with all the acrimony of pole- 

 mical controversy. 



Some of the opinions, however, expressed in these works with 

 respect to the laws that govern volcanic action, were severely criticised 

 at the time. Others have been since opposed by rival theories. And, 

 as these disputed questions have an important bearing on some of 

 the most interesting problems of geology, I trust it may not be un- 

 profitable to call the attention of our Society to the more prominent 

 among them. 



I will advert on this occasion to two subjects especially, viz. 



I. The origin, or mode of formation, of volcanic cones and craters. 



II. The nature of the liquidity of lava at the time of its protrusion 



from a volcanic aperture. 



I. Formation of Cones and Craters. — In both of the works to 

 which I have alluded, I referred the formation of those remarkable 

 circular hollows, usually called craters, which are of such frequent 

 occurrence in volcanic districts, to explosive aeriform eruptions, 

 breaking their way through the superficial rocks ; and that of the 

 external more or less conical hill or mountain which generally, but 

 not always, environs a crater, — and which, indeed, often occurs with- 

 out a crater, but always characterized by the qua-qaa-versal dip of 

 its constituent beds of lava and conglomerates, — to the accumulation, 

 round and above an eruptive vent, of its fragmentary ejections and 

 the lava-streams poured out from it. 



I considered this law to be without exception; attributing the 

 differences in figure and structure apparent among volcanic cones to 

 the greater or less number and violence of the eruptions to which 

 they were owing, — some being the product of a single eruption, 

 others of a vast number, often repeated through a series of ages, — 

 to differences in the position of the orifices of discharge, whether 

 from the summit of the cone, or its base, or any intermediate points, 

 — and whether from under water, or in the air, — to the varying 

 mineral character of the products, — and to the influences of subse- 

 quent degradation. 



At the same time I remarked that the earthquakes which always 

 more or less accompany volcanic eruptions render probable a certain 



