444 



NA TURE 



[Marc/i 12, 1885 



as expected, and were unable to pass beyond the glacier alleged to 

 obstruct the Copper or Atna River, about sixty miles from the sea. 

 Meanwhile, a party has actually started, under Gen. Miles's 

 orders, January 30, for the Copper River, consisting of Sergeant 

 Robinson and F. W. Ficket, signal-observer U.S.A., and 

 commanded by Lieut. Allen. They intend to go to the mouth 

 of the Atna or Copper River by steamer, and ascend as far as 

 possible on the ice, pushing on by water as soon as the ice 

 breaks up and the freshets are over. They hope to cross the 

 divide from the Upper Atna, and descend by one of the Yukon 

 tributaries to the mouth of the latter river, and rejoin civilisation at 

 St. Michael's. They may be fortunate enough to make the journey 

 in one season, but are prepared to stay two years. They will add a 

 number of Indians to the party at Sitka, and carry various peace- 

 offerings for the Atna Indians. Lieut. Stoney, of the navy, is 

 reported to have a new expedition nearly organised to continue 

 his investigations of the Kowak River. The plan adopted, so 

 far as yet decided upon, is to take a steam-launch, ascend the 

 river as far as possible, and pursue the explorations to its source, 

 and winter in the region if necessary. It is stated that the 

 party is to be composed of sixteen men, which is dangerously 

 large, considering the limited food-resources of the region, and 

 might be advantageously diminished by one-half for explorations 

 in the interior. If the party were to pass over the divide, and 

 investigate the course of the Colville, returning via Point Barrow 

 next summer, it would accomplish a praiseworthy and much- 

 needed investigation. 



We have received from Messrs. W. and A. K. Johnston a 

 school physical wall map of England and Wales, in which the 

 altitudes above sea-level are shown by varieties of tint. Of its 

 kind this map is good, though we should prefer to see the 

 method of tints combined with the graphic method, in order that 

 pupils may be taught to read the maps with which they have to 

 deal when they become men and women. Accompanying the 

 map is a little hand-book of the physical geography of England 

 and Wales. 



THE SCOPE AND METHOD OF 

 PETROGRAPHY 1 

 TN considering the history of geology we are struck by the 

 fact that towards the close of the last and during the com- 

 mencement of the present century, when the science was taking 

 rank as an important branch of human knowledge, petrography 

 occupied a much higher position than it has at any subsequent 

 period. 



Werner, whose influence was almost unrivalled at the time to 

 which I have referred, was a mineralogist, and his formations 

 were therefore naturally based on the mineralogical characters 

 of the different rocks. His observations were limited for the 

 most part to his own district of Saxony, but he regarded his 

 formations as sediments or precipitates from a universal ocean, 

 and his numerous pupils, fired by his love of science and his 

 intense enthusiasm, rejoiced in extending his d:.s lfication to the 

 districts with which they were severally acquainted. 



The magnificent v, ork of those who devoted special attention 

 to the organic remains in the sedimentary deposits, and espe- 

 cially that of William Smith, the "Father of British Geology," 

 had the effect of deposing petrography from the position which 

 it held under the influence of Werner and his followers. It was 

 clearly shown that the fossil contents of the strata were far more 

 reliable as evidence of chronological relations than their litho- 

 logical characters, and as soon as this became generally recog- 

 nised, the reduction of the fossil-bearing rocks all over the 

 world to something like definite order followed as a natural 

 consequence. 



The principle that strata may be identified by means of their 

 fossil contents has not only proved applicable to the Secondary 

 and Tertiary formations to which it was originally applied by 

 Smith, Cuvier, and others, but it has been extended by Murchi- 

 son, Sedgwick, Barrande, and others to the older rocks. Speaking 

 broadly, there can be no doubt that over large areas the suc- 

 cession of the forms of marine life has been remarkably uniform 

 from the Cambrian times down to the present, so that we have 

 in the fossil contents of the different strata by far the most re- 

 liable means of determining chronological relations. 



It is not surprising, then, that petrography has been compara- 

 nd Tte3oil!^TG.S. n the W00dwardian Museum, Cambridge, by J. J. 



tively neglected by geologists, for their main object during the 

 present century has been to classify the stratified rocks which 

 form so large a portion of the existing land surfaces. 



At the present time, however, we are witnessing a great re- 

 vival of interest in petrography, not only in this country but all 

 over the globe. This is due in part, no doubt, to the introduc- 

 tion of new methods of research ; but it seems to me that there 

 are other and more general causes. The clear recognition of the 

 great principle with which the name of William Smith is so 

 indissolubly united at once made it possible for a host of obser- 

 vers to do excellent work in every quarter of the globe. The 

 interest awakened by the study of the geological structure of the 

 most densely populated regions was akin to that which is felt by 

 the geographical explorer of unknown lands. Until the main 

 features of the geology of fossiliferous regions were described, it 

 was not to be expected that observers would turn aside from a 

 field of research in which they were certain to meet with success 

 for the purpose of attacking problems which, after all, might 

 prove to be insoluble. As time went on, the unexplored tracts 

 in which fossiliferous rocks occur became more and more re- 

 stricted, or more and more inaccessible, and when the old chaos 

 of Grauwacke fell into order before the brilliant researches of 

 Sedgwick, Murchison, and Barrande, geologists were placed in 

 an entirely new position. They had conquered that portion of 

 the world which was open to their special method of attack. A 

 number of fortresses still held out, it is true, and many of these 

 remain unsubdued at the present day. They will doubtless 

 occupy the attention of those who are most skilled in the old 

 methods of warfare for many years to come. At the same time 

 I think it will be admitted on all hands that the brilliant suc- 

 cesses of the old generals have left a large portion of the army 

 with little to do. We must, therefore, look for other worlds to 

 conquer. 



Now, on taking a general survey of the subject-matter of 

 geology it will be seen at once that we are profoundly ignorant 

 on questions relating to the origin and sequence of volcanic 

 rocks, the cause or causes of volcanic action, the mode of forma- 

 tion of the crystalline schists, and the origin of mountains. That 

 these questions are really unsolved is proved by the difference 

 of opinion which exists between 'competent observers. Another 

 point which strikes one is, that if a solution of these problems 

 be ever realised, it will be due in a great measure to the com- 

 bination of field geology and petrography. This, it seems to 

 me, will explain the great interest which is taken in the latter 

 branch of science at the present day. If I am right in my 

 opinion as to the present state of things, then we may safely 

 predict that petrography will occupy as prominent a position in 

 the immediate future of geology as palaeontology has done in 

 the past. In making this statement I trust it will not bethought 

 that I am claiming too high a position for that branch of geo- 

 logy with which I am most intimately acquainted. 



Let us turn now to a more detailed consideration of the scope 

 and method of petrography. The rocks of the earth's crust 

 form the subject-matter of the science. Now these may be 

 studied from two more or less distinct points of view — the de- 

 scriptive and the etiological. We may set to work to describe 

 the rocks, that is, to ascertain and record every possible fact 

 with regard to them ; or we may endeavour to trace the succes- 

 sion of events which has culminated in the state of things which 

 we actually observe. It is perfectly obvious that we cannot 

 hope to attain any considerable success in the second branch of 

 the subject until we have devoted a considerable amount of 

 attention to the first. 



Descrip'ive petrography then concerns itself with the chemical, 

 mineralogical and physical characters of the individual rocks, and 

 also witli the distribution and mutual relations of the different 

 varieties. The last-mentioned branch of the subject occupies 

 the same position in petrography as comparative anatomy does 

 in zoology. It may therefore be termed comparative petrography. 



When the history of the science comes to be written, it will be 

 recognised that it is to the Germans we are especially indebted 

 for our knowledge of descriptive petrography. The amount of 

 work which has been done in Germany is immeasurably greater 

 than that produced by other nations. For years past they have 

 been steadily improving their methods of observation, as well as 

 observing and recording facts. Moreover, they have been train- 

 ing petrographers who are now scattered all over the world. The 

 Americans especially have availed themselves of the laboratories 

 of Rosenbusch and Zirkel, and almost every Annual Report of 

 the American Survey now bears witness to the influence of 



