Noz. lo, 1887] 



NATURE 



41 



the remaining fourteen distinctions are accomplished by varia- 

 tions of tone. While it may be possible to select twenty-four 

 hues available for indiscriminate combination, there can be no 

 question that those provisionally printed by the Committee will 

 fail to maintain their distinctness when variously combined upon 

 a map. Under the influence of such chromatic environments as 

 are sure to be encountered, the four yellow hues of the Tertiary 

 cannot be discriminated, and the same difticully will arise with 

 the two hues of gray assigned to the Carboniferous, and with 

 the hues of gray and brown assigned respectively to the Permian 

 and the Devonian. Some of the tones likewise are not suffi- 

 ciently distinguished. Two of the blues of the Jurassic, two of 

 the browns of the Devonian, two of the rose tones of the 

 Archaean, and the two violets of the Tria<, are open to this 

 criticism. A certain amount of adjustment can be made in the 

 final selection of inks, and probably all the defects from tone 

 can be thus remedied, but the confusion of hues is more difficult 

 to eliminate, for the great number of the hues interferes with 

 the separation of those that are too approximate. To strengthen 

 one contrast is to weaken another. 



In order to judge of the availability of the scheme for the 

 production of detail maps, it is necessary to consider the resolu- 

 tions of the Congress as well as the printed legend. A resolu- 

 tion provides that the subdivisions of a system shall be repre- 

 sented by shades of the colour adopted for the system, or by 

 broken colour or other texture devices; and if is further provided 

 that the shades, whether produced by solid colour or by texture, 

 shall be so arranged that the darkest or strongest represent the 

 lower divisions of the system. The resolution is in French, and 

 the word I have translated shade {nuance) is one which applies 

 popularly to either hue or tone, while in the scientific termino- 

 logy of chromatics it applies to hue only. The Committee on 

 the map has taken it in its popular sense, and has represented 

 some subdivisions by hues, and others by tones ; for example, 

 Pliocene and Miocene are assigned two tones of the same hue, 

 while Oligocene and Eocene have each a separate hue. The 

 Upper Cretaceous and part of the Lower Cretaceous are assigned 

 a green hue in two tones, while the Gault and the Wealden, 

 classed as subdivisions of the Lower Cretaceous, have independ- 

 ent hues of green. Of the six reds assigned to volcanic rocks, 

 two agree in hue and differ in tone, while the remainder have 

 distinct hues. As the legend stands, both major and minor 

 distinctions — that is to say, the discrimination of groups, the 

 discrimination of systems, and the discrimination of divisions 

 smaller than systems — are all accomplished by differences of 

 hue ; v\ hile the discrimination of minor divisions is accom- 

 plished indifferently by variation of hue and by variation of 

 tone. The same means performs several functions, and the 

 same function is performed by several means. 



It is stating the same thing from another point of view to say 

 that the Congress and its Committees have used the term colour 

 in its popular rather than its scientific sense. Scientifically, a 

 colour is a particular tone of a particular hue, and the number 

 of colours is infinite. Popularly, a colour is an assemblage of 

 contiguous hues and their tones, to which a name has been given. 

 Each hue and tope within the range covered by the name is a 

 shade of the colour. It is in this popular sense that the resolu- 

 tions assign a colour to each system, and assign shades of the 

 system-colour to the subdivisions of the system. 



Now, if in the variation of a system-colour, by textures or other- 

 wise, a single hue is adhered to, the system-colour remains 

 distinct from other system-colours throughout all its modifica- 

 tions and their modifications ; but if hues as well as tones are 

 varied, the inevitable result is confusion, for some of the hues of 

 one system-colour will approach too near to hues of other 

 system-colours. With a multiplicity of minor distinctions the 

 main distinction of system from system will be lost. 



Another difficulty lies in the fact that the Quaternary and 

 Devonian colours, while strongly contrasted in tone, are nearly 

 identical in hue. This does not affect their use in a general map, 

 but in a detail map the stronger tones of the Quaternary gray 

 will approach too closely the paler tones of the Devonian brown. 



These criticisms aj^ply to those features of the scheme which 

 affect its adoption for general and detail maps of European 

 countries. There is one of equal or greater importance affecting 

 its application in other continents. It is adjusted to the rock 

 systems of Europe exclusively, and makes no provision whatever 

 for the systems of other parts of the earth. The geologists of 

 Wisconsin, for example, cannot use it without calling the 

 Keweenawan either Cambrian or Archaean. If they were in 



doubt which division should hold it, but inclined a little oneway 

 or the other, they could express their qualified opinion in the 

 notation provided by the Map Committee ; but having attained 

 an unqualified opinion that the terrane belongs to neither of these 

 two categories, they find no means for expressing their conclu- 

 sions. The scheme cannot be applied to the geology of India, 

 of New Zealand, or of Australia, without misrepresentation. It 

 is not universal but local, and this because it is founded on the 

 fallacy of a world-wide unity of geologic systems. 



So far as the geology of the world is concerned, it would be 

 better to adopt no convention at all as regards map colours than 

 to adopt one carrying with it and promulgating a vicious classi- 

 fication. Uniformity is not worth purchasing at the price of 

 falsification. If the members of the Congress cannot agree upon 

 a plan having the flexibility demanded by the geologic facts, it 

 will be best to limit its action to the local problems involved in 

 the map of Europe. I believe, however, that the necessary 

 flexibility is attainable ; and before proceeding to further criticism 

 of the Committee's scheme, I will give the outlines of a plan 

 which appears to me to combine the advantage of flexibility with 

 a number of other desirable qualities. 



The plan is founded on the universality of geologic time and 

 the diversity of local geologic histories as expressed in rock 

 systems. Geologic periods are arranged in linear order. Each 

 one adjoins the next, and together they constitute continuous 

 geologic time, which we may conceive as represented by a straight 

 line. The stratigraphic systems of a country have likewise an 

 order of succession, and their arrangement is linear. They are 

 not always continuous one with another, but the history recorded 

 by the systems and the breaks between them is continuous, and 

 may be represented by a straight line, equal and parallel to that 

 of geologic time. And so for each country. A colour scale 

 which shall represent each and all of these parallel lines must be 

 itself lintar and continuous, and fortunately we have such a scale 

 furnished us in the prismatic spectrum. 



I propose, first, that the continuous prismatic spectrum be 

 adopted as the standard universal scale for continuous geologic 

 time. I propose, second, that the conventional time scale, based 

 on the geologic history of Europe, be complemented by a colour 

 scale, prismatic but discontinuous. I would assign to each period, 

 not a certain portion or area of the spectrum, but a specific colour 

 defined by its position in the spectrum. This colour scale will 

 also apply to the geology of Europe. I propose, third, that the 

 students of each geologic district .shall as ign to the stratigraphic 

 systems of that district a set of prismatic colours so selected from 

 the spectrum as to properly represent the relation of each system 

 to the time scale, provided that relation is approximately known. 

 Under this rule a system corresponding partly with the Cretaceous 

 and partly with the Jurassic will receive a prismatic colour inter- 

 mediate between those assigned to the Cretaceal and Jural divi- 

 sions of the time scale. I propose, fourth, that systems whose 

 relations to the standard time scale are not even approxiniately 

 known be given tentative positions in the time scale and assigned 

 the corresponding colours ; and that such provisional colours be 

 distinguished by a special device. 



Of this device I will speak later, but before we leave this part 

 of the subject the capability of the plan to express the facts 

 should be more clearly characterized. Continuous geologic 

 time being equated with the continuous spectral band of light, 

 each period is theoretically equated with a segment of that band 

 including all the hues between certain limits. But practically 

 the period is represented in the colour scale only by the central 

 hue of the segment, and there is nothing in the nature of this 

 hue to indicate the length of the segment. Similarly each local 

 system is represented only by the hue corresponding to the 

 middle of the equivalent period, considered as a part of the con- 

 tinuous time scale, and this hue gives no information as to the 

 magnitude of the system or the duration of the correspondmg 

 period. When a non-European system is represented on a map 

 with the Devonal colour, all that is expressed is that the middle 

 of its period coincides with the middle of the Devonal period ; 

 the whole period may equal the Devonal or may be shorter or 

 may be longer. With this limitation the scheme is able to 

 express the exact facts, or the exact state of opimon, m regard 

 to correlation. 



I propose, fifth, that the subdivisions of systems be repre- 

 sented, if their number is small, by distinct tones of the hue 

 assigned to the system, and if their number is great, by mono- 

 chromatic textures. It having been provided that sy>tems shall 

 be distinguished by means of hues, it is now provided that hues 



