BUNSEN MEMOKIAL LECTURE. 627 



which he therein expressed mark an era in the history of geoloo-inil 

 theory. It is now acknowledo-od that the idea which lie was th<'1irst 

 to propound, namely, the necessity of examining- the chemical compo- 

 sition of eruptive rocks taken as a whole rather than the determination 

 of their various constituent minerals must be carried out if wc wish 

 to come to an understanding as to their mode of formation. For this 

 purpose he made an extensive series of complete analyses of the Ice- 

 landic rocks. And from these results he drew the remarkable con- 

 clusion that in Iceland, and probably in most of the lai-ger volcanic 

 systems, there exist two extreme types of rocks. One of these, riche.«*t 

 in silica, is termed the "normal trachyte;" the other, containing less 

 silica and naturally more basic constituents, is the "normal pyroxene.'' 

 All the Icelandic rocks can be classed as being either one or other of 

 these normal silicates, or as admixtures of the two. In order to 

 account for these well-established facts, Bunsen supposed that the two 

 normal types were separated out from the mass of molten silicate in 

 the interior of the earth at distinct points; and he founded this sup- 

 position on the fact of the influence of pressure on the melting ])(nnt. 

 (Pogg. Ann., 1850 (81), 562.) 



This had been independentl}' pointed out by James Thomson, in 

 1849, as being a corollary of the mechanical theory of heat, and had 

 also been experimentally verified by William Thomson (Lord Kelvin) 

 in the case of water. Bunsen developed this point further by proving 

 that, exposed to a pressure of 156 atmospheres, the temperature of 

 solidification of spermaceti was raised from 47.7-, under ordinary 

 atmospheric pressure, to 50.9°. As volcanic rocks must have bt>en 

 subjected to varying pressures amounting to many thousands of atmos- 

 pheres, it is clear that the effect of such variation on the point of 

 solidification of the rocks must be very considerable, and that where 

 the pressure is less the composition of the crvstalline mass would b«> 

 different from that of the rock formed where the pressure is gn^atei-. 

 This remarkable theory of the existence of two distinct types of rocks 

 separating out from the same fluid mass has recently been supi>lanted 

 by other views, but the facts respecting the composition of the eruptive 

 rocks upon w^hich the idea was based will ever remain not only a 

 monument to the patience and perseverance of their discov«'rer. but 

 as some of the most valuable additions to our knowledge of clieinical 

 geology. 



Bunsen's investigations of the pseudo- volcanic phenoineiia ' of 

 Iceland, and especially those of the great geyser, may. indeed, be con- 

 sidered as models both from a piiysical and a chemical i)oint <.f view. 

 The temperature experiments, which were both difficult and dangerous, 



'An excellent English translation of the memoir "On the intimate .umuM-tinn 

 between the pseudo-volcanic phenomena of Iceland" in f..m.d u. the volume of 

 Reports and Memoirs printed by the Cavendish Society in 1848. 



