444 G. K. Gilbert — Congress of Geologists. 



printed in the report of the Bologna meeting afford, with thejr 

 manifest permutations, about two hundred distinct textures, and 

 I am satisfied from a study of these and others that as many as 

 one hundred can be chosen that are not subject to confusion. 

 It follows that a map or atlas expressing few distinctions need 

 use only hues, or only hues and tones, but where numerous 

 distinctions are to be made, recourse must be had to textures. 



The printing of a large number of textures of the same hue 

 produces a greater number of tones than can be discriminated, 

 and its effect is to confuse and nullify any distinctions (within 

 the range of that hue) based purely on tone. The printing of 

 a large number of bichromatic textures causes the same result, 

 and it also produces a greater number of hues than can be dis- 

 criminated ; its effect is to confuse and nullify distinctions based 

 purely on tone, or on hue, or on tone and hue together. 



In the color scheme prepared for the map of Europe thirty- 

 eight distinctions are made. There are twenty-four hues, and 

 the remaining fourteen distinctions are accomplished by varia- 

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

 hues - availiable 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 environ- 

 ments as are sure to be encountered, the four yellow hues of 

 the Tertiary cannot be discriminated, and the same difficulty 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 sufficiently 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 Trias 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 diffi- 

 cult 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 reso- 

 lutions of the congress as well as the printed legend. A reso- 

 lution provides that the subdivisions of a system shall be rep- 

 resented by shades of the color adopted for the system, or by 

 broken color or other texture devices, and it is further pro- 

 vided that the shades, whether produced by solid color or by 

 texture, shall be so arranged that the darkest or strongest rep- 

 resent the lower divisions of the system.* The resolution is in 



* Congres geologique international, Compte rendu de la 2me Session, Bologne, 

 1881, p. 157. 



