March 3, 1887] 



NATURE 



417 



same rock-mass, the species of minerals which are present 

 and their proportions to one another may, and, indeed, 

 often do, vary from point to point. Nor does minute 

 structure, though affording admirable criteria for distin- 

 guishing certain types of rock, supply a sufficiently definite 

 means of diagnosis for all the difterent varieties which 

 occur. A system of " lithology " may, indeed, be devised, 

 if we confine our attention to the hand-specimens in our 

 museums ; but it breaks down the moment that we attempt 

 to apply it in our researches in the field. 



I have long felt assured that all attempts at a nomen- 

 clature and classification of rocks must, for the reasons 

 just staled, be regarded as tentative and provisional only ; 

 but the careful study of rock-types is nevertheless bringing 

 to light a number of facts calculated to profoundly modify 

 mineralogical no less than geological thought and specula- 

 tion. 



Petrology forms the link between mineralogy and 

 geology, just as palaeontology does between biology and 

 geology. Mineralogy has justly been styled the alphabet 

 of petrology ; but if the orthography and etymology of the 

 language of rocks lie in the province of the mineralogist, 

 its syntax and prosody belong to the realm of the 

 geologist. In that language, of which the letters are 

 mineral species and the words are rock-types, I am per- 

 suaded that there is written for us the whole story of 

 terrestrial evolution. 



Petrology, it is clear, could make but little progress 

 until the improvement of microscopic methods enabled 

 us to make accurate determinations of the minerals in a 

 rock, even when these are present as the most minute 

 particles. The characteristic peculiarities of the different 

 rock-forming minerals, so carefully studied by Zirkel, their 

 accurate optical diagnosis, at which Rosenbusch has 

 laboured with so much success, these with the micro- 

 chemical methods of Knop, Boficky, Streng, and Behrens, 

 and the pyro-chemical method of Szabo, have already 

 done much to render exact our methods of recognising 

 the minerals in a rock. The contrivances, for which we 

 are principally indebted to the French petrographers, for 

 effecting the isolation of the minerals in rocks, so that 

 they may be submitted to accurate chemical analysis, 

 enable us in cases of difficulty or doubt to confirm or 

 check the results of our microscopical studies. 



But there is at present, perhaps, a tendency to confound 

 the end with the means in such researches as these. When 

 all the varieties of minerals in a rock have been correctly 

 identified, the work of the petrologist is not ended ; on 

 the contrary, it is only just begun. 



The relationship of the several minerals in a rock to 

 one another, the discrimination between such as are 

 original and those of secondary origin, and the recogni- 

 tion among the former of the essential, as distinguished 

 from those that are accessory or accidental, — these are 

 problems of even greater importance than the exact 

 determination of the species or varieties to which each 

 belongs. In not a few rocks it can be demonstrated that 

 every one of its present mineral constituents is different 

 from those of which it was originally made up ; in some 

 cases, indeed, it may be shown that the recombination of 

 the elements of the rock into fresh mineral aggregates has 

 taken place again and again. As well might we try to 

 give a rational account of our English speech without 

 taking into account the series of changes through which 

 it has passed in its evolution from the Anglo-Saxon 

 dialects, as to explain the nature of a rock without study- 

 ing the influence upon it of the forces by which it has 

 gradually acquired its present characters. 



With respect to the geographical distribution of the 

 different mineral species, many suggestive obser\-ations 

 have been made. Some, like the feldspars, the pyroxenes 

 and the olivines, appear to be ubiquitous in our earth's 

 crust, and even make their appearance again in those 

 bodies of extra-terrestrial origin — the meteorites. Others, 



like leucite, nepheline, hauyn, sodalite, and melilite, are 

 exceedingly abundant in certain areas of the earth's 

 surface, while they appear to be wholly wanting in 

 others. 



Still more remarkable arc the relations which are found 

 to exist between the types of rocks occurring in different 

 geographical areas. The study of this subject is leading 

 us to the recognition of the fact that there are distinct 

 petrological provinces. In closely adjoining areas— such 

 as Hungary and Bohemia, for example— widely different 

 types of rock have been erupted during the same geolo- 

 gical period ; and this is a fact not less striking and 

 significant than that of the meeting of two perfectly dis- 

 tinct biological provinces along a line which traverses the 

 Malayan archipelago. It cannot be doubted that the 

 prosecution of this hopeful branch of study— the geo- 

 graphical distribution of minerals and rocks— will lead 

 us to results of the highest interest and value. 



That there will be shown to be a distribution of rocks 

 in time, as well as in space, I am perfectly prepared to 

 believe. I cannot but think, however, that some of 

 the generalisations on this subject which have been 

 hazarded are somewhat premature. To a geologist (espe- 

 cially one belonging to the school of Lyell) it is equally 

 difficult to conceive that there should be a broad distinction 

 between the metamorphic rocks of Archaean and post- 

 Archeean age respectively, as that the pre-Tertiary vol- 

 canic rocks should be altogether different from those of 

 Tertiary and recent times. 



The great object of al! our studies — concerning the 

 morphology, the physiology, and the chorology— of the 

 mineral kirigdom, ought to be to arrive at definite ideas 

 concerning its jetiology ; the causes by which the existing 

 forms, capabilities, and positions of minerals and rocks 

 have been determined. 



While the /o.r«7j contained in rock-masses afford us the 

 means for determining the date of their origin, the careful 

 study of the minerals which they include may enable us 

 to unravel the complicated series of changes through 

 which they have passed since their first formation. 



Eighteen years ago, when seeking to show how 

 the origin of a particular rock might be elucidated by a 

 combination of studies in the field, in the chemical 

 laboratory, and by the aid of the microscope, I ventured 

 to offer to this Society some general remarks on this sub- 

 ject. As it has been my constant endeavour since that 

 time to apply the principles then enunciated in the case 

 of rocks of more complicated character and more recon- 

 dite origin, I may perhaps be forgiven for repeating the 

 words I then used. Every rock since its first formation 

 " has undergone and it still is undergoing a constant series 

 of internal changes, the result of the action of difterent 

 causes, as heat, pressure, solution, the play of many 

 chemical affinities, and of crystallographic and other 

 molecular forces, causes insignificant perhaps in them- 

 selves, but capable under the factor time of producing 

 the most wonderful transformations. The geologist is 

 called upon to unravel the complicated results, to pro- 

 nounce what portion of the phenomena presented by a 

 rock is due to the forces by which it was originally formed, 

 and what must be referred to subsequent change ; to dis- 

 criminate the successive stages of the latter and to detect 

 their various causes ; in short, to trace the history of a 

 rock from its deposition to the present moment." 



Dr. Wadsworth has well characterised the changes 

 which take place in rock-masses as due to the tendency 

 of unstable mineral combinations to pass into stable ones. 

 It must be remembered, however, that stability is a rela- 

 tive term, and that the arrangement of molecules which 

 is stable under one set of conditions becomes unstable 

 under another set. As by tlie internal movements and 

 the external denudation of the earth's crust, the conditions 

 under which rock-masses exist are undergoing slow but 



