NOT ALL LAVAS AEE PEODUGT OF EBFUSION. 121 



approximate identity of chemical constitution is what we should anticipate 

 on that assumption. We should expect to find some minerals common to 

 both classes of roc'ks, while other minerals are found in one class alone. 

 We should look for nothing but contrast in the respective mechanical tex- 

 tures ; and we find the anticipated agreements and contrasts. 



But there is an important consideration which will not permit us to 

 conclude that all eruptive rocks are derived from the fusion of metamor- 

 phics ; for whence came the materials of the metamorphic rocks them- 

 selves 1 Accepted theories declare that their ultimate origin was in the 

 primordial materials of the earth-mass, which were broken up, decom- 

 posed, and the several components sorted out and arranged in the form of 

 sediments ; and these sedimentary formations gradually accumulated until 

 they completely buried the primordial mass, so that no portion of it is 

 anywhere exposed, so far as has yet been discovered. But when the prim- 

 itive mass was finally buried, from what sources could the materials have 

 been derived which could add fresh layers to the covering? To this there 

 is but one possible answer. After the greater portion of the original sur- 

 face had been covered, additional sediments must have been derived from 

 the extravasation of primordial matter. This conclusion seems to be logi- 

 cally perfect. In the past epochs these primitive materials must have been 

 continually extravasated, though, as the body of sedimentary formations 

 increased, it is possible that they too began to be erupted by secondary 

 fusion, and with the lapse of time formed an increasing proportion of the 

 total extravasation, while the proportion of primitive matter as gradually 

 diminished. Now, have we any reason for supposing that the evolution of 

 the earth has so far advanced that primitive matter has ceased to erupt, and 

 that modern outbreaks consist wholly of materials which had once before 

 in the world's history been poured out, broken up, decomposed, stratified, 

 metamorphosed, and again erupted 1 If so, then the body of stratified rocks 

 is no longer increasing, but the revolutions of time are simpl} T working 

 over the stratified rocks again and again. But this is improbable in a high 

 degree. There is no warrant whatever for such a belief, and therefore no 

 justification for the inference that all eruptive rocks are derived from the 

 secondary fusion of the metamorphics. But if it is probable that some of 



