Notice of a Geological Model. 85 
extent. Both the branches alluded to are bounded or enclosed > 
by corresponding mountain ridges; the,strata of which, composed 
of the inferior red shales and of a numerous series of sandstones 
and conglomerates, underlie the coal measures and the upper red 
shales. he coal strata in these separate branches or basins dip, “a 
for the most part, to their respective centres. 
The horizontal area is protracted as a scale of two inches to a 
mile; the data for which have been derived from a variety of 
public and local surveys. In the vertical scale we have been en- 
abled to approach so near to the horizontal as two of the former 
to one of the latter; an approximation which is more close than 
is usual in such works. And here it may be permitted to apply 
some remarks on the construction of diagrams. 
It has been customary with most geologists, and I believe almost 
universally with civil engineers, where the bases of their sections 
are considerably extended, to adopt a much larger scale for the 
perpendicular than for the longitudinal dimensions. Consequent- 
ly, the diagrams, so drawn, amount to absolute distortions, and 
manifestly convey very inaccurate ideas. The proportions of 
relative heights and lengths are thus so grossly caricatured, that 
they bear but distant resemblances to what is intended to be rep- 
resented. ‘The inclinations of strata are changed from moderate 
angles almost up to vertical; the altitudes of hills are stretched 
to the eminence of lofty peaks; rounded dary i 
ires; gentle u 
surface of a S thus met- 
assume the form of atten 
craggy steeps, and the o 
amorphosed into a region ha 
alpine crests, and steep and fathomless gulfs—a hideous burlesque 
upon the actual aspect of the district represented, or rather mis- 
represented. ' 
In constructing geological diagrams I have, for some time, 
ceased to make any difference between the horizontal and verti- 
cal scales. At any rate I have endeavored, as closely as may be, 
to adhere to that principle. If the drawings be executed with 
delicacy they rarely require a deviation from the rule; and I 
would respectfully recommend an adherence to it, among my ge- 
ological friends, particularly in relation to the state surveys, where 
comparisons of sections are continually needed. We shall then, 
and not till then, possess something like uniformity in the repre- 
sentations o similar things. So long as the distorting principle 
= 
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arts 
