62 Notices 7-especting New Books. 



Secondary actions, moreover, are often set in motion in conse- 

 quence of the infiltration of heated water, or of vapours ; and these 

 actions may extend to some distance from the actual intrusive mass. 



He divides the intrusive* rocks into lavas, trappean rocks and 

 granitic rocks ; and he examines the action of each upon the various 

 rocks which include or surround them, as the ores, the combustible 

 rocks, the felspathic rocks, and calcareous, siliceous, and argil- 

 laceous rocks. 



The first conclusion which M. Delesse states as the result of his 

 investigations, is that the metamorphic effect of mere heat has been 

 greatly exaggerated ; and he then gives an account of the action of 

 heat on some substances, as observed in the laboratory and in sub- 

 terranean fires in mines. 



We must say that the conclusions to be drawn from these experi- 

 ments, as well as those derivable from the observation of the action 

 of lava-streams on rocks, do not appear to us to be necessarily 

 applicable to the action of deeply-seated rocks, without much 

 caution and reserve. The conditions of pressure, and its action on 

 the volatile constituents of certain rocks ; the possible more or less 

 complete exclusion of atmospheric air at great depths, and the 

 difficulty of excluding it from rocks heated in laboratories or mines, 

 and the possibly indefinite differences in the rates of cooling in the 

 two cases, require, we think, to be taken into account to a much 

 larger extent than they usually are. We examine a granitic or 

 trappean dyke and the rock it traverses at the present surface of the 

 ground, or in some mining gallery near the surface, and we often 

 forget that at the time that dyke was injected there was perhaps 

 5000 or perhaps 10,000 feet of rock or more above what is now 

 the surface. 



From Mr. Sorby's recent admirable researches, indeed, it appears 

 probable that in the case of many granite veins, instead of 5 or 10, 

 we ought more probably to suppose 50 or 60 thousands of feet to 

 have been incumbent on the rocks at the time of the injection of 

 the granite. 



M. Delesse gives several careful descriptions, however, of locali- 

 ties where trap-dykes were injected into rocks that were, at all 

 events, not so deeply buried at the time of injection. Several of 

 these places are in the County Antrim, where a vast mass of inter- 

 stratified basalt and " tuff" rests upon the chalk. He describes, at 

 one place near the Giant's Causeway, a bed of lignite as covered by 

 a bed of trap, and so little altered that it is impossible, according to 

 him, that the trap could have been a molten mass when it flowed 

 over the lignite. He also gives a detailed description of the dyke 



* M. Delesse, as is usual on the Continent, uses the term "eruptive" 

 instead of intrusive ; as, however, the word eruptive seems to involve the 

 idea of the rock reaching the surface, and as there can be little doubt that 

 all the granitic rocks appear at the surface only in consequence of vast 

 subsequent denudation, and were originally cooled and consolidated at 

 great depths, it appears to us better to use the term " intrusive" for 

 all the igneous rocks. 



