D. Forbes — On the Study of Chemical Geology. 3G7 



British geologists seem to have all but exclusively devoted them- 

 selves to the consideration of the stratigraphical and palgeontological 

 succession of tlie sedimentary beds, and have, as a rule, studiously 

 avoided the investigation of all geological phenomena which did not 

 appear to admit of explanation by the agency of mere mechanical 

 forces. At the same time, however, it is curious to observe that 

 there have not been wanting those who have put forth vague theories 

 to account for the nature and formation of our metamorpbic and 

 eruptive rocks, etc., hypotheses which, unfortunately, can only be re- 

 garded as flights of imagination, since it is well known that, with 

 but some few rare exceptions, no chemico-geological investigations 

 or chemical analyses of British rocks or of their component minerals 

 have as yet been made which could serve as a basis for any such 

 generalisations. 



foremost as this country is in all the other departments of geology, 

 Mr. Forbes considers Great Britain to be far behind in Chemical 

 Geology. 



The author cautions his readers against the innate tendency 

 to take up a favourite cause or hypothesis, to which is often 

 attributed eifects in reality the result of some very different 

 agency, or to the combined action of several causes ; the student 

 should, therefore, be particularly careful not to attach himself to any 

 special theory or school of geology which might bias him when 

 estimating the value of evidence brought forward on any question 

 under consideration. 



He then proceeds to point out the great importance of a careful 

 definition of the terms igneous, aqueous, and gasolitic action, when 

 applied to the study of geological phenomena. 



1. Igneous action is the action of heat as seen developed in active 

 volcanoes, the study of which led to the formation of the Plutonic 

 school of geologists. This is not a mere dry fusion, like melting lead, 

 glass, or other anhydrous substances in a crucible, but is one in 

 which, whilst heat plays the grand role, is in nature invariably ac- 

 companied by the action of the vapour of water and gases. 



2. Aqueous action is the action of waters (fresh or saline) such as 

 are seen on the present surface of the globe ; and is not the mere 

 solvent action of pure water, but is one in which the air, gases, salts, 

 and other bodies contained in natural waters, assisted by heat, materi- 

 ally alters the solvent powers and chemical reactions of the water 

 itself. 



3. Gasolitic action is the effect of gases and vapours, more or less 

 assisted by heat. 



All these agencies are naturally modified by the effects of chemical 

 action and mechanical force. In all three cases, the actions of heat, 

 of water, and of gases are found to be combined, each playing a 

 more or less prominent part. Yet there can be no misunderstanding 

 or confusing the precise meaning to be attached to each term. 



In igneous or volcanic action, whilst the effects of heat pre- 

 dominate, the presence of heated steam and gases exercises a most 

 important influence in modifying the results : and in this case the 

 water present is in the form of steam. 



