NATURE 



475 



THURSDAY, OCTOBER 4, 1877 



MICROSCOPICAL PETROGRAPHY 

 Microscopical Peiiography. By Ferdinand Zirkel. Being 

 Vol. VI. of the Report of the United States Geological 

 Exploration of the Fortieth Parallel made under the 

 direction of the Engineer Department by Clarence 

 King, Geologist-in-charge. (Washington, 1876.) 

 ''T^O the massive and important series of volumes in 

 J- which the Report of the Exploration of the Fortieth 

 Parallel has been published the Engineer Department of 

 the United States has just added a sixth which, for 

 general interest and usefulness beyond the area of the 

 Survey, is equal if not superior to any that has preceded 

 it. In the course of this protracted and laborious survey 

 many rocks were encountered to which Mr. Clarence 

 King and his coadjutors felt somewhat at a loss to apply 

 the petrographical nomenclature of Europe. He accord- 

 ingly sought help from the highest European authority on 

 the subject, Prof. Zirkel, of Leipzig, whom he induced to 

 undertake the task of examining the vast collection of 

 rock-specimens which had gathered during so many 

 years of field-work. Prof. Zirkel accordingly crossed the 

 Atlantic, spent some time in New York with Mr. King 

 and his stall in making a preliminary investigation of the 

 collection, and in learning the geological position of the 

 specimens and the geological structure of the wide region 

 from which they had been obtained. Subsequently a 

 large and typical series of rock-specimens was sent over 

 to Leipzig to be submitted to careful microscopical inves- 

 tigation. No fewer than twenty-five hundred thin sec- 

 tions were prepared and examined under the microscope. 

 The result of Prof. Zirkel's laborious task is now given to 

 the world and most appropriately forms a separate volume 

 of the Report on the Geo'ogy of the Fortieth Parallel. 

 Mr. King miy be congratulated upon the judgment he 

 has sho*n in the allocation of his miterials. He has 

 enriched his official publications with the most important 

 contribution yet made to the petrography of America. 



Of the way in which Prof. Zirkel has acquitted himself 

 of the task he undertook, it is hardly possible to speak 

 too highly. With the characteristic method of his coun- 

 trymen he marshals his facts in such orderly fashion that 

 every observation has its appropriate and proper place 

 where it may be expected and where, if sought for, it 

 will be found. Familiar as he is \\\\\\ the minute texture 

 and composition of most European rocks, it must have 

 been a congenial, even .though laborious work, to attack 

 o"! such a scale those of another continent. He has evi- 

 dently given himself heartily to the investigation, and has 

 produced a work which more than sustains his well- 

 earned reputation. 



In an introductory chapter the author briefly sketches 

 the leading types of microscopic structure which, largely 

 as a result of his own previous labours, have been recog- 

 nised among crystalline rocks. These may be reduced 

 to three : — i. The purely crystalline, that is, rocks which 

 display only crystals or crystalline particles so interwoven 

 as to form a solid, compact mass. Granite may be taken 

 as the type of this group. 2. The half-crystalline. Rocks 

 of this group cons'st partly of crystals or crystalline 

 Vol. XVI. — No. 414 



particles, and partly of a non-crystalline amorphous sub- 

 stance or paste, which may be (rz) a colourless but more 

 usually yellow, brown, or grey glass ; {b) partly devitrified by 

 the appearance of minute translucent but nonpolarizable 

 grains (globulites), or variously-shaped opaque needles or 

 hairs (trichites) ; {c) still further devitrified by the in- 

 crease of these grains and needles, so that little or no 

 glass remains— a structure termed micro-crystallitic ; or 

 (((') a peculiar amorphous substance neither showing the 

 transparency of glass nor definite grains and needles 

 (crystallites), but appearing to consist of indistinct grains 

 or fibres, which seem to melt into each other. This is 

 termed the microfelsitic. 3. The non- crystalline. Here 

 the rocks consist sometimes merely of glass, as ob- 

 sidian, sometimes of the amorphous microfelsitic sub- 

 stance, as in felsites. Dr. Zirkel admits, however, that 

 even where these differences of minute structure are best 

 shown they do not suffice as a basis for the systematic 

 arrangement of rocks, which must rest on fundamental 

 mineral constitution. The same mass of rock, indeed, 

 may within a short space put on extraordinary diversities 

 of minute structure. 



A number of terms are introduced into the Report 

 which, though most of them have for some time been in 

 use in Germany, for the most part make their first 

 appearance here in an English dress. " Ground mass " 

 is employed to denote what seems to the naked eye to be 

 the dense homogeneous matrix of a rock, wherein the 

 usual scattered porphyritic crystals are held ; " base " is 

 used as the designation of what is only seen under the 

 microscope to be a non-crystallised or unindividualised 

 paste, glassy, globulitic, micro-crystallitic or micro-felsitic, 

 as the case may be, in which the crystals, whether micro- 

 scopic or visible to the naked eye, are held. " Macro- 

 scopic " has obtained wide currency in German petro- 

 graphical literature as a convenient designation for what 

 can be seen without the use of lenses. " Microlites" are 

 minute, thin, needle- shaped, usually cylindrical bodies, 

 which occur both in the base and in separate crystals of 

 rocks, and represent imperfect stages in the crystallisation 

 of different minerals ; wh^n colourless they are called 

 " belonites," when black and opaque, " trichites." 



As most rocks have undergone more or less internal 

 alteration, many products of decomposition are met 

 with under the microscope which cannot always be 

 identified with definite mineral species. No one who 

 has practically studied microscopic petrography can fail 

 to have been often puzzled to name some of these pro- 

 ducts. They are in far too minute quantity and too 

 intimately dilTused through the substance of a rock to 

 be capable of being collected for chemical analysis. 

 They present no recognisable crystallographic form, and 

 they show no distinctive reaction under the polariscope ; 

 yet they have too often, with no expression of hesitation, 

 been identified with known minerals, the identifications 

 being at the best only guesses, and sometimes most 

 improbable ones. It has lately been the practice at 

 Leipzig to avoid attempting such identifications when the 

 evidence is so slight, but to be content with the applica- 

 tion of provisional names which may include many 

 different compounds having at least some common 

 characters, such as opachy or colour, and to wait until 

 the progress of invcitigation allows more precise names 



