42 



NA TURE 



\_Nov. lo, 1887 



shall have no other function. This secures the integrity of 

 the distinction between systems, whatever the minuteness ot 

 subdivision. 



The idea of using the spectral colours in their proper order is 

 not novel. It has entered into half the plans submitted to the 

 Congress, but each author has introduced other colours also, or 

 else has undertaken to use the spectrum colours more than once, 

 under the impression that they do not afford the necessary range 

 or variety. This impression is based largely upon the popular 

 meaning of the word colour. It is indeed true that if we limit 

 ourselves to those parts of the spectral series which have uni- 

 vocal names, we have only six or seven distinctions ; and it is 

 further true that if we have recourse to binomial designations, 

 such as yellowish green and greenish yellow, we obtain rather 

 indefinite conceptions ; but to men of science there are better 

 i-esources than those afforded by the language of e very-day life. 

 The spectrum has been elaborately studied, and the relations of 

 its dark lines to its colours have been determined. Its wave- 

 lengths have, moreover, been measured, and by such means as 

 these we are furnished with three different scales, any one of 

 which is adequate to the precise definition of any hue of the 

 continuous series. What needs to be done is this. When 

 the divisions of the time scale have been decided on, the 

 spectrum must be studied lo ascertain the best selection of 

 hues. Their number must, of course, be that of the number 

 of divisions of the time scale, and they must be so chosen 

 that the degree of separateness of adjacent colours shall be 

 everywhere the same, as judged by the normal human eye. 

 Then define each hue by its wave-length or its position in the 

 Kirchhoff scale, and define it also in terms of the best com- 

 bination of pigments with which it can approximately be repro- 

 duced for practical use. It is, of course, impossible to copy 

 the prismatic colours with accuracy, because the colours of pig- 

 ments are impure, but this difficulty will not seriously interfere 

 with the employment of the prismatic colours as a standard. 



The practical question whether the spectrum will give a 

 sufficient number of hues so far separated from each other as 

 to be distinguishable in all the arrangements occurring on 

 maps has received such consideration as I have been able to give 

 it, and it is my judgment that the maximum number of hues that 

 can safely be used falls somewhere between fifteen and twenty. 

 There will certainly be no difficulty in thus constructing a 

 standard colour scale with about a dozen terms. 



The employment of the spectral colours in this manner leaves 

 three groups of colours unassigned — the purples, the browns, 

 and the grays. If the spectral colours be arranged on the cir- 

 cumference of a circle so that each diameter of the circle connects 

 hues that are complementary, it is found that they occupy the 

 greater part, but not quite all, of the circumference, and the 

 colour needed to fill the vacant arc is purple. The hues of 

 purple might then, if deemed necessary, be added to one end or 

 the other of the spectrum, thus increasing the range from which 

 to select colours for the time scale. 



My sixth proposition is to assign the browns to volcanic rocks. 

 I would leave the grays unassigned. 



It will be observed that no intimation has been given as to 

 whether the violet end of the spectrum should apply to the 

 newest system of strata or the oldest. It must of course be 

 definitely assigned to one or the other, but the particular 

 assignment is a matter of indifference. 



The main features of the proposed prismatic scheme have now 

 been set forth, and you are fairly entitled to exemption from the 

 minor features, but there is one detail that can hardly be omitted. 

 In one of the main propositions it was provided that some 

 special device should distinguish colours assigned to uncorrelated 

 systems, and I feel it incumbent to show .that a suitable device 

 can be found. Of a number that have occurred to me as about 

 equally available, I will mention but a single one — the over- 

 printing, in small dots, widely separated, of the complementary 

 colour. The complementary colour is selected because it does 

 not disturb the relation of the system- colour to the colours of 

 adjacent systems. Bichromatic over-printing produces a hue 

 intermediate between the two hues combined, but the hue mid- 

 way between a system-colour and its complementary colour is 

 white or gray, and if only a small amount of the complementary 

 colour is added, the system-colour merely becomes paler or 

 duller, when viewed from such a distance that the colours blend. 



The prismatic colour scheme, having been constructed for the 

 express purpose of securing a degree of flexibility that will fit it 

 for universal use, need not be further compared in that regard 



with the scheme published by the European' Map Committee. 

 Enough has also been said to show that its superior perspicuity 

 is claimed both for general and for detail maps. A few words 

 will suffice to compare the two systems in other respects. 



As regards the expense incun-ed in the production of general 

 maps, neither has any notable advantage, and they are not yet 

 sufficiently developed to permit a comparison as regards the cost 

 of detail maps. Their capability for the production of pleasant 

 colour effects can best be judged when maps have been actually 

 made, but it may be said in a general way that the Committee's 

 scheme will afford more strong contrasts between adjacent colour 

 areas than the prismatic. The maps coloured l^y the former will 

 be relatively lively, those coloured by the latter relatively quiet. 

 It is provided by the Committee that the volcanic colours shall 

 be not merely red but strong. On a general map volcanic areas 

 cover comparatively small spaces, and strong reds thus disposed 

 will ordinarily add brilliancy ; but the detail map of a volcanic 

 district, thus coloured, will be disquietingly suggestive of active 

 eruption. 



The alphabet of colours for the prismatic scale will be the 

 more easily learned of the two, because it is orderly, and because 

 its order is already familiar in the spectrum. The Committee's 

 scheme, however, has some old-fashioned mnemonic features 

 which the prismatic lacks. The green of the Cretaceous is con- 

 nected with greensand, the red of volcanic rocks with fire, and 

 the rose of the Archaean with feldspar ; and the gray of the 

 Carboniferous mildly suggests the blackness of coal. 



In respect to facility of introduction the Committee's scheme, 

 being essentially a compromise of existing colour scales, has the 

 advantage that to most users it is not entirely novel. The pris- 

 matic scheme on the other hand has the advantage of being 

 orderly. It scientifically differentiates the functions of hues and 

 tones, and though each one of its colours may be different from 

 what the individual geologist has previously employed for the 

 indication of the same system, the order of the colours is already 

 familiar to him in another way. 



This closes my review of the various works undertaken by the 

 Congress. Some of these have been favoured, others opposed, 

 and reasons have been given. But there is a general considera- 

 tion or criterion applicable to all, which has nearly escaped 

 mention, although it is of pre-eminent importance. When a 

 matter is proposed for regulation by the Congress, the first 

 question which should be asked is whether it falls within the 

 legitimate purview of a convention of geologists. It manifestly 

 does not if it belongs to some other science rather than to 

 geology, and objection has on this ground been made against 

 the regulation by our Geologic Congress of the nomenclatures 

 of palaeontology and mineralogy. But not all geologic matters 

 even are properly subject to settlement by convention. This is 

 peculiarly the case with geoljgic facts. Science is distinguished 

 from the earlier philosophies of mankind by the peculiarity that 

 it establishes its fundamental data by observation. The old 

 philosophies were founded largely upon assumptions, and it was 

 not deemed illogical — perhaps it was not illogical — to appeal to 

 the authority of an assemblage of experts for the establishment 

 of fundamental assumptions. But for science it is not merely 

 illogical, it is suicidal, to establish facts in any other way than 

 by observation. No vote of the most august scientific body can 

 possibly establish a fact, and no vote can have any weight 

 against a good observation. 



Now the entire science of geology, using the phrase in a strict 

 sense, is constituted by the aggregation and arrangement of 

 facts, and none of its results can be rendered more true or be 

 more firmly established, or be prevented from yielding to contra- 

 dictory facts, by conventiinal agreement. A classification, if it 

 has any value whatever, is merely a generalized expression of 

 the facts of observation, and is outside the domain of the voter. 

 If it comprises all the essential facts, its sufficiency will eventually 

 be recognized, whether its authority is individual or collective. 

 If it does- not comprise them, it will inevitably be superseded, 

 by whatever authority it may have been instituted. For this 

 reason I am opposed to the classification by the Congress of the 

 sedimentary formations, and likewise to the classification of 

 volcanic rocks, and I also regard it as ill-advised that the Con- 

 gress undertook the preparation of a map of Europe, for that — 

 if more than a work of compilation — is a work of classification. 



If we examine the other undertakings of the Congress — the 

 definition and gradation of taxonomic terms, the i-ystematization 

 of terminations, the selection of a scale of colou:s for geologic 

 maps, and the selection of other conventional signs for the 



