294 THE EVOLUTION OB" PETEOLOGICAL IDEAS. 



"A short time ago it seemed as if the Neptunean theories had com- 

 pletely abandoned the field in favor of those volcanic theories which 

 appeared so absurd to our ancestors. Now the Neptunean theories are 

 beo-innini>- to show sions of life. I have endeavored to conciliate tiie 

 two sister enemies by suogestino- that water may play an important 

 part in the formation of fused rocks. * * * I do not prc^tend, 

 however, that my theory is unassailable, or that it has been a])solutely 

 demonstrated. * * * In my opinion a geological theory should 

 not be considered as absolute; but it becomes probable when a consid- 

 erablo number of facts group themselves around it, and its degree of 

 probal)ility can be measured l\y its power of assimilating the new facts 

 brought to light by the progress of science.'" 



These are wise words, and may well be remembered when differ- 

 ences of opinion tend to become sharply accentuated. The path of 

 science is littered with discarded theories, and this fact should serve to 

 remind us that "we are none of us infallible, not even the youngest." 



In the discussion between Scheerer and Durocher attention was 

 directed to the mutual relations of the constituents of granite and to 

 the inferences Avhich could })e drawn from a study of those relations 

 as to the order of consolidation of the minerals. 



THE CONSOLIT>ATTON OF IGNEOUS MAGMAS. 



At the time when the discussion took place this kind of reasoning 

 could ])e applied oidy to coarse-grained rocks, l)ut with the advent of 

 the microscope it became possible to extend it to the important group 

 which had l)een designated in the earlier classitications as "apparentl}^ 

 homogeneous rocks.'' In the early part of the century Cordier had 

 proved, by the microscopic examination of the powder of l)asalt, that 

 this rock was heterogeneous; but it was not till the examination of 

 thin sections had been introduced that the nuitual relations of the con- 

 stituents of the liner-grained rocks could ))e studied. In those rocks 

 which have resulted from the consolidation of homogeneous silicate 

 magmas, and in which the consolidation has been unaccompanied by 

 the plieiiomcMia of resorption — that is, in which there has always been 

 ecpiilibrium between tho constituents during the process of consoli- 

 dation — the order of separation can l)e inferred from the microscopic 

 struct ui'(\ 



It has thus beiMi estal)lished that the process of consolidation can )iot 

 be divided into a number of sharply defined periods, each characterized 

 by the separation of some one miiKM'al only, but that the times during 

 which the diU'erent minerals are s(>pa rating out overlap to some extent. 

 The amount of overlapping varies in different cases, and in one and 

 the same magma is most marked in ])lutonic masses; whence we con- 

 clude that it is largely determined by physical conditions, and especially 

 pressure. 



The order of consolidation as determined by an examination of the 

 mutual relations of minerals is, therefore, the order in which they com- 



