G. K. Gilbert — Congress of Geologists. 449 



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 assign- 

 ment 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 omit- 

 ted. In one of the main propositions it was provided (hat some 

 special device should distinguish colors assigned to uncorrel- 

 ated 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 overprinting, in small dots, widely separated, of the comple- 

 mentary color. The complementary color is selected because 

 it does not disturb the relation of the system-color to the colors 

 of adjacent systems. Bichromatic overprinting produces a hue 

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

 way between a system Color and its complementary color is 

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

 ary color is added, the system -color becomes merely paler or 

 duller, when viewed from such a distance that the colors 

 blend. 



The prismatic color 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 incurred in the production of general 

 maps, neither scheme has 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 color effects can be best 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 color areas than the prismatic. The maps colored by 

 the former will be relatively lively, those colored by the latter 

 relatively quiet. It is provided by the committee that the vol- 

 canic colors shall be not mereljr 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 colored, will be dis- 

 quietingly suggestive of active eruption. 



