March 12, 1885] 



NA TURE 



445 



Germany from a teaching point of view on the growth of petro- 

 graphical science. In this sketch, of course, I am only calling 

 attention to the broad facts of history as far as regards the special 

 branch of descriptive petrography. Many observers in France, 

 England, and America have done independent work of the very 

 highest order, and to England especially belongs the credit of 

 having, in the person of Sorby, determined to a very large 

 extent the introduction of the modern methods of microscopical 

 research. 



Consider now what is involved in the description of any par- 

 ticular rock, and take, for example, a specimen of the Whin Sill, 

 that mass of basic igneous rock which plays such an important 

 part in the Carboniferous region of the North of England. 



The rock is dark gray or bluish-gray when freshly exposed. 

 In texture it varies from compact to coarsely crystalline, the 

 most common variety being one in which the individual con- 

 stituents are just recognisable by the naked eye. Its specific 

 gravity varies from 2'CjO to 2'95. Its chemical composition is 

 shown on this table. (Table referred to. ) We have now to 

 consider its mineralogical composition. In the determination of 

 minerals in rocks we use physical and chemical methods. Colour, 

 general appearance, hardness, cleavages, specific gravity, crys- 

 talline form, fusibility, and flame coloration are some of the 

 most important physical properties available for the determin- 

 ation of minerals in rocks when they can be examined macro- 

 scopically. In thin sections we can use colour, general appear- 

 ance, cleavages, form, and also the many properties which are 

 brought out by the use of parallel and convergent rays of polarised 

 light. Chemical tests may be applied both to macroscopically 

 recognisable minerals and also to those which can only be 

 observed by the use of thin sections or minute particles and the 

 microscope. The latter a r e generally referred to as micro- 

 chemical tests. 



By applying these methods, some of which will be more fully 

 explained in the subsequent lectures, we can prove that the rock 

 of the Whin Sill is composed of felspar, pyroxene, titanifcrous 

 magnetic iron ore, quartz in the form of grains and also as a 

 constituent of micro-pegmatite, apatite, pyrite, brown horn- 

 blende, mica, and some green decomposition products. Apatite, 

 pyrite, hornblende, and mica occur only in very small quantity, 

 and cannot be said to form any appreciable portion of the rock. 



In order to give a complete petrographical description, how- 

 ever, it is necessary that we should not only know what minerals 

 are present, but also that we should know the precise composi- 

 tion of each and the relative abundance of the different species. 

 Information on these points cm only be obtained by isolating 

 the different constituents of a rock and analysing them sepa- 

 rately. Methods of isolation will be described in subsequent 

 lectures. The most important are those which depend on the 

 use of heavy solutions, the magnet and electro-magnet, and 

 various chemical reagents, especially hydrochloric and hydro- 

 fluoric acids. The chemical composition of each of the three 

 principal constituents of the Whin Sill is represented on these 

 tables. (Tables referred to. ) Now, having obtained a know- 

 ledge of the composition of the principal constituents of the 

 rock, it becomes possible to determine with a very fair amount 

 of accuracy the relative proportions of these constituents by 

 calculations based on the bulk analysis of the rock. 



There is yet another point of great importance to which atten- 

 tion should be paid in subjecting a rock to an exhaustive 

 examination. Owing to the brilliant researches of Sandberger, 

 it is beginning to be recognised that many of the heavy and so- 

 called rare metals are present in ordinary rocks in minute 

 quantities. Until recently we have been disposed to regard 

 these substances as occurring only in mineral veins and in the 

 deeper portions of the earth from which the mineral veins have 

 been supposed to derive their supply of material. Nov? il is 

 beginning to be clearly recognised that these substances are very 

 widely distributed even in the superficial crust of the globe. As 

 an illustration of the interest and practical importance of the 

 subject above referred to I may call attention to the important 

 work by Dr. Becker, on the " Geology of the Comstock Lode," 

 recently published by the U.S. Geological Survey. This lode, 

 the yield of which is supposed to have sensibly affected the 

 bullion markets of the world, occurs in a region which is re- 

 markable for the extreme development of igneous rocks (diabase, 

 diorite, andesites, &c), and for the widespread alteration to which 

 these rocks have been subjected. The bisilicates, especially, 

 have been affected by this alteration, and for immense distances 

 they have been entirely replaced by a green chloritic mineral. 



Most careful assays have been executed, under the super- 

 vision of Dr. Becker, for the purpose of determining the 

 amount of bullion in the fresh and unaltered rocks, and the 

 relative amounts of gold and silver. He says : " By com- 

 parison of the different assays it appears that decomposed dia- 

 base carries somewhat less than half as much silver as the fresh 

 rock. When the decomposed rocks are pyritous, the experi- 

 ments made do not indicate any essential diminution of the gold 

 contents. This fact, however, is quite possibly due to irregu- 

 larity in distribution and the minuteness of the quantities of gold 

 to be determined. As the decomposition of the rock in question 

 has proceeded at a great depth beneath the surface, it is highly 

 unlikely that silver should have been extracted unaccompanied 

 by gold. Much of the decomposed rock, too, is nearly free 

 from pyrite, and had the gold contents of such specimens been 

 determined, a smaller percentage would probably have been 

 found. The omission [to select specimens free from pyrite] was 

 not detected until it was too late to resume the investigation. 

 So far as quantitative relations are concerned, the silver only 

 can be relied on, though the qualitative detection of gold as well 

 is both interesting and important." 



Another point of great interest was determined by Dr. Becker. 

 He isolated the felspar and the augite of the diabase and tested 

 both from silver. He found that for equal weights the augite 

 was eight times as rich as the felspar substance, and as the latter 

 contained some augite, this appears to furnish substantial proo 

 that the silver is a constituent of the augite. 



Having subjected a rock to exhaustive chemical and minera- 

 logical examination, it next hecomes necessary to compare it 

 with ' ther rocks, and to give it a name. The subject of nomen- 

 clature is a very difficult one. It is much to be regretted that 

 notwithstanding all that has been done in descriptive and com- 

 parative petrography, we are still far from having any system of 

 classification which is capable of general acceptance. Indeed, 

 we are not agreed as to the first principles on which a 

 classification of rocks should be based. The German "petro- 

 graphers, in most cases, adopt at the outset a principle 

 which we cannot accept. They divide igneous rocks into 

 older and younger : the former including all those which 

 they regard as pre-Tertiary, the latter all those which are of 

 post-Cretaceous age. The division is based, of course, on the 

 assumption that the conditions of eruption in"pre-Tertiary times 

 were essentially different from those which have prevailed since. 

 There seems, so far as we can judge, little or no ground for this 

 assumption. The few facts which do at first sight lend support 

 to it appear to be equally capable of explanation on the other 

 hypothesis. The typical pre-Tertiary rocks of the German 

 system are the granites, diorites, gabbros, diabases, and 

 syenites. Now there is reason to believe that these are all 

 plutonic rocks ; in other words, that they are the result of slow 

 consolidation beneath the surface, and therefore under great 

 pressure. If this view be correct then their exposure at the 

 surface can only occur long after their formation, and the fact 

 that the majority of those known to us should be of pre-Tertiary 

 age, as Lyell long ago pointed out, need occasion no surprise. 



Again, it must be remembered that the mere association of 

 eruptive rocks « ith pre-Tertiary deposits is no proof in itself that 

 the former are of pre-Tertiary age, and also that many competent 

 observers believe that these are clear cases of Tertiary granite, 

 diorite, diabase, and gabbro. 



The igneous rocks, which are regarded by the German petro- 

 graphers as especially characteristic of the post-Cretaceous period, 

 are the basalts, andesites, trachytes, and rhyolites ; in other words, 

 the surface-products of volcanic action. That these should be 

 mainly Tertiary, and that they should differ to a certain extent 

 from their pre-Tertiary equivalents in consequence of alteration, 

 is only what might be naturally expected. This, however, is 

 not sufficient to justify the refusal to give the same name to 

 different specimens of one and the same rock merely because 

 they have been produced at different periods ; and the work of 

 Airport, Bonney, Geikie, and others has proved that there are 

 basalts, andesites, and rhyolites of Palaeozoic age which are 

 identical in structure, composition, and mode of occurrence with 

 modern rocks. 



In the absence of any generally recognised system of nomen- 

 clature it becomes difficult to assign a name to the rock of the 

 Whin Sill. It is a holo-crystalline rock composed essentially 

 of plagioclase, pyroxene, titaniferous and magnetic iron ore. 

 In sections the felspar occurs in lath-shaped forms. To such a 

 rock, provided it be of pre-Tertiary age, Rosenbusch would 



