294 Prof. F. Lcewinson-Lessing — Origin of the Igneous Hocks. 



from these mixtures there have originated those accidental rocks which 

 can be designated, after Harker, as hybrid rocks. 



§ 3. On the Origin of the Igneous Rocks. 



Let us now consider the third fundamental problem of petrogenesis. 

 What origin have the igneous rocks, and why are they represented by 

 the same types in all periods ? 



Different geological data and theoretical considerations lead to the 

 conclusion that the identity of the eruptive rocks of all geological 

 periods can be most satisfactorily explained by the assumption that 

 from the Archaean up to the present the eruptive rocks represent 

 nearly the same material, which has been subjected several times 

 to weathering, metamorphism, refusion, and regeneration. The 

 assumption that we find or can find anywhere the primordial solid 

 crust of the globe must be definitely abandoned. The occurrence 

 of clastic and sedimentary rocks in the oldest Archaean formations 

 and numerous examples of refused and recrystallized rocks in these 

 formations eloquently sustain and complete the theoretical con- 

 siderations which lead us to the conception that the primordial 

 crust has been re-melted long ago, and probably more than once. 

 Such a fusion of parts of the solid crust is sometimes directly 

 attributable to a rising of the isogeotherms at the places in question. 

 In reality the process may be a more complicated one, as can be 

 shown by the following considerations. A series of sedimentary 

 rocks 30,000 feet thick (the greatest thickness we can admit) would 

 simply by the rising of the isogeotherms acquire in the lowest beds 

 a temperature of 300°, which is quite insufficient for melting 

 these materials. But the geosyncline, where sedimentation of our 

 30,000 feet of sediments has taken place, may itself consist of an 

 older series of sedimentary material. A.t a depth of 60,000 feet under 

 the bottom of our geosynclinal the temperature would be 600° C. 

 before sedimentation, and would rise to 900° C. after the deposition of 

 30,000 feet of sediments. Under the weight of this new sedimentary 

 sheet of 30,000 feet the area in question would probably bend and 

 subside; this would give for a presumable subsidence of 10,000 feet 

 a further increase of 100°, and so the primary temperature would in 

 this way rise from 600° to 1,000° C. All these figures are, of course, 

 hypothetical. But we must bear in mind that epeirogenetic and 

 orogenetic movements can occasion a far greater subsidence of 

 parts of the crust than the process of sedimentation by itself. It is 

 also not to be forgotten that in these great depths the magma 

 is probably rich in water and different gases, and consequently 

 fusible at a lower temperature than the ' dry ' magmas generally 

 considered. And, lastly, we must also infer with Suess that the 

 fusion of certain parts of the earth's crust may be produced partly 

 by the rising from below of hot plutonic gases. In short, there 

 are sufficient factors for sustaining the hypothesis that in successive 

 geological periods different parts of the solid crust have been caused 

 to melt, and that by this process have been generated the plutonic 

 and volcanic rocks. 



