GEOLOGY. 253 



North America. The Maclurea is indeed known in the Silurian 

 limestone of the south of Scotland ; but the Ophileta and other forms 

 are not found until we reach the horizon of North America. Now, 

 these fossils refer the zone of the Highland limestone and associated 

 quartz-rocks to that portion of the lower Silurian which forms the 

 natural base of the Trenton series of North America, or the lower 

 part of the Llandeilo formation of Britain. The intermediate forma- 

 tion the Lingula flags or "zone primordiale " of Bohemia hav- 

 ing no representative in the north-western Highlands, there is 

 necessarily a complete unconformity between the fossil-bearing crys- 

 talline limestone and quartz-rocks with the Maclurea, Murchisonia, 

 Orphileta, Orthis, Orthoceratites, etc., and those Cambrian rocks on 

 which they rest. 



A- great revolution in the ideas of many an old geologist, including 

 myself, has thus been effected. Strengthened and confirmed as my 

 view has been by the concordant testimony of Ramsay, Harkness, 

 Geikie, James, and others, I have had no hesitation in considering a 

 very large portion of the crystalline strata of the Highlands to be of 

 the same age as some of the older fossiliferous Silurian rocks, whether 

 in the form of slates in Wales, of graywacke-schist in the southern 

 counties of Scotland, or in the conditions of mud and sand at St. Pe- 

 tersburg. Many years ago, I suggested, after examination, that some 

 of the crystalline rocks near Christiana, in Norway, were but altered 

 extensions of the Silurian deposits of. that region ; and since then the 

 truth of the suggestion has been demonstrated. We all know, fur- 

 thermore, that in North America all noted geologists, however they 

 may differ on certain details, agree in recognizing the fact that the 

 vast eastern seaboard range of gneissic and micaceous schists is made 

 up of metamorphosed strata, superior even to the lowest of the Silu- 

 rian rocks. In regard, however, to the modus operandi by which 

 whole regions of sedimentary deposits have been converted into crys- 

 talline slates and schists, we are far from having satisfied our minds. 

 The Rev. W. Hare ourt, .after a series of trials to illustrate the phe- 

 nomena of the metamorphism of rocks by experiments carried on in 

 iron furnaces, has arrived at the same conclusion at which, in com- 

 mon with Sedgwick, Buckland, De la Beche, Phillips, and others in 

 my own country, and with L. Von Buch, Elie de Beaumont, and a 

 host of geologists abroad, I had long ago arrived in the field. I, 

 therefore, re-echo their voices in repeating the words of Mr. W. Har- 

 court, "that we are not entitled to presume that the forces which 

 have operated on the earth's crust have always been the same." 

 Looking to the only rational theory which has ever been propounded 

 to account for the great changes in the crust which have taken place 

 in former periods, the existence of an intense central heat, which 

 has been secularly more and more repressed by the accumulation of 

 sediment, until the surface of the planet was brought into its present 

 comparatively quiescent condition, Mr. Harcourt has indicated the 

 train of causes, chemical and physical, which resolve some of the dif- 

 ficulties of the problem. 



Illustrating his vieAvs by reference to chemical changes in the 

 rocks and minerals of our own country, and fortifying his induction 

 by an appeal to his experiments, he arrives at the conclusion that 



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