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differences in adjoining rocks by strong opposition of hue; to avoid 
spottiness ; and lastly, to apply the brightest colours to the centre, 
carrying them off by gradation towards the extremities. All these 
objects, Mr. Greenough states, can rarely be attained in any case, 
but all were taken into consideration before the colouring of any 
portion was finally decided upon. 
He then alludes to the attention which is necessary in the ap- 
plication of the pigments, and the difficulties of securing unifor- 
mity of tint as well as correctness of boundaries; also to the ne- 
cessity of employing artists of acknowledged skill and established 
character. 
As the accomplishment of the above requisites requires great care, 
and is necessarily attended with considerable expense, Mr. Greenough 
enters at some length into the application of machinery to geological 
mapping. He shows, that by the employment of shaded grounds 
the number of tints may be diminished, and colours not be required to 
do more than they can perferm ; and that where the shade is produced 
by lines repeated at small intervals, there is obtained a tint cheaper, 
purer, brighter, more constant and uniform, more durable, and far 
less injurious to the engraving beneath than any wash. ‘The breadth, 
depth, and mutual distance of the lines being all capable of modifi- 
cation, a large series of such lines may be obtained, varying from the 
faintest grey to absolute black, every one of which being neutral, 
must harmonize with whatever colour may happen to be next to it. 
The direction of the lines also may vary indefinitely ; and a further 
resource may be obtained in the use of dots; and the form of these 
again may be varied indefinitely, When engraved lines or dots are 
employed, not as single tints, but as a eround for other. tints, they 
have the great merit of subdividing a formation without destroying 
its unity. Mr. Greenough availed himself of this expedient in laying 
down the beds of the Weald and of the Cotswolds. 
The preparing of an uniform system of tints, and inducing its 
adoption, Mr. Greenough states is not very easy of execution on the 
old plan; first, because the number of substances to be distinguished 
varies indefinitely, according to the nature and object of the several 
maps in which the uniform system is to be employed; secondly, be- 
cause different minds frame different schemes of classification, and 
enlarge or diminish the groups as their own good pleasure or the na- 
ture of the countries represented may suggest ; thirdly, because the 
same colour employed under different conditions, would lead to very 
different optical results, being in perfect harmony when applied to one 
map, when applied to another, painfully discordant. 
Though the list of formations in elementary books is not long, yet 
the number of divisions and subdivisions which require admission into 
geological maps very constantly exceeds the number of tints which 
the most practised eye can discriminate; nor is it expedient, under 
any circumstances, that these tints should be repeated. 'To over- 
come this difficulty, Mr. Greenough proposes the combination of co- 
lours with linear shadows; and he indulges a hope that it will not be 
found difficult, by the judicious application of this simple contrivance, 
