Notice of a Geological Model. 85 



extent. Both the branches aUuded 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. The coal strata in these separate branches or basins dip, 

 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 secondary mountains 

 assume the form of attenuated spires ; gentle undulations become 

 craggy steeps, and the ordinary surface of a country is thus met- 

 amorphosed into a region harshly broken into pinnacled spires and 

 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 of similar things. So long as the distorting principle 



