ANXIYEESART ADDKESS OP THE PRESIDENT. lui 



phism, taken from the British Islands, and often described from 

 personal inspection, render this volume indispensable to those who 

 desire to follow out the useful teachings of Macculloch, or to consider 

 with attention the theoretical views of Hutton and Lyell. 



Distinguishing in the first place between the case of " normal 

 metamorphism," operated on a vast scale, and by the means of a 

 general pervading agency, in which heat, if not the sole cause, is 

 the great excitant, M. Delesse determines, in some specially favour- 

 able cases, the probable temperature to which strata have been 

 exposed through contact with heated plutonic rocks. It is well 

 known that the effects on carbonaceous rocks which have been sub- 

 mitted to this metamorphism by contact are the loss of volatile parts 

 and the production of coke — often in prismatic forms as regular as that 

 of consolidated lava. The temperature necessary for these effects is 

 found to be lower than that often assumed by geologists for the 

 effects of this order, and not to exceed 400° C.= under 800° F., 

 somewhat below red heat. Supposing the increment of temperature 

 to be 1° C. for 33 metres, this heat would be met with at 13,200 

 metres, say 40,000 feet. It does not appear to M. Delesse that the 

 effect of metamorphism by heat is much modified by mere depth, 

 for the changes effected by subterranean fire, when they can be 

 accurately noted, confirm this ; and, by consequence, rocks which 

 have been really submitted to intense heat have always a certain 

 character stamped on them by which they can be recognized. Thus 

 the study of metamorphism by contact embraces two branches — the 

 changes in the rock which encloses the once fluid agent, and the 

 alterations which this itself experiences. 



To follow M. Delesse through the large collection of examples 

 which he has amassed for discussion, and which he has discussed, 

 generally with the aid of careful chemical analysis, would be a 

 pleasing but unnecessary task. In clear simple language, the results 

 of the whole inquiry are summed up under the heads of effects on 

 the enclosing rock, according as the metamorphic agency is volcanic, 

 trappean, or granitic, and effects on the eruptive rock. 



The effects produced by lava are incontestably due to mere heat, 

 while those observed in the contacts with trap indicate often an 

 action of heated liquid solutions, and often the effect of a rock 

 heated and plastic, but not really fused as lava is seen to be. So 

 granite appears to have become often plastic, and yet not really 

 fused, and the changes which it occasions are not such as mere heat 

 can produce. Here we find M. Delesse apparently taking a view of 

 the once plastic nature of granite and trap, not unlike that pre- 

 sented by Mr. Scrope in regard to the flowing of recent lava. 



Mr. Sterry Hunt has recently communicated to the Society a 

 memoir treating generally of the processes by which sedimentary 

 deposits may have been transformed into crystalline rocks. Among 

 other instances by which he is induced to reject great heat as the 

 cause of the metamorphism of sedimentary rocks, he mentions " un- 

 oxidized carbon in the form of graphite, both in crystalline lime- 

 stone and in beds of magnetic iron-ore ; and it is well known that 



