301 
In the course of the summer they also made (with similar objects) 
several traverses through the Hartz, and one long traverse from the 
Thuringerwald to the north flank of the Fichtelgebirge, in the hope 
of bringing into relation with their previous observations, the country 
which has become so celebrated from the labours of Count Munster. 
The authors follow this order in the descriptive parts of their 
paper. But before commencing their detailed sections, they explain 
at some length the method of determining the order of superposition 
among rocks, like those of Belgium and the Rhenish provinces, 
which are not only much contorted, but often in a reversed position. 
This order of superposition can be made out only by sections, which 
are of two kinds, vertical and horizontal. Vertical sections, where 
the strata are not inverted, not only indicate the natural group of 
strata, but their true order of superposition, both of which may often 
be ascertained on a single line of traverse. But horizontal sections, 
showing the intersection of successive groups of strata with the ac- 
tual surface of the country (and represented by the colours of a 
geological map), can only be examined by following the lines of 
strike. The colours of such a section (if derived from strata origin- 
ally conformable) must show the masses, however contorted, in their 
true juxtaposition. Hence we may define from the horizontal sec- 
tions of a country a true consecutive geological series; and if the 
relative age of any two contiguous terms be known, the relative age 
of all the other members of the section may be inferred with cer- 
tainty, though the formations be in an inverted position, as seen on 
the line of any one vertical section. It was by this laborious method 
of “horizontal sections,” or, to use his language, by determining the 
symmetry of position of the several formations, that Professor Du- 
mont first disentangled the perplexing phznomena of the Belgian 
provinces. The authors, after acknowledging the great value of 
this principle, state that they endeavoured never to lose sight of it 
in estimating the value and interpreting the meaning of the many 
vertical sections they examined in their long traverses through the 
provinces they describe. 
§ 1. Coal-fields of Westphalia, c.—The authors commence their 
descending sections, on the right bank of the Rhine, with the pro- 
ductive coal-field, which occupies an irregular triangular area, 
bounded towards the north by greensand and cretaceous deposits, 
towards the south-east by older formations, afterwards to be de- 
scribed, and towards the south-west by an irregular line, skirting 
the low country near the Rhine, and passing near Mulheim, Ketwick, 
Werder, and thence to a point a few miles north-east of Elberfeldt. 
In its lithological character and fossil contents it is not to be di- 
stinguished from the coal-fields of England. It is affected by many 
anticlinal and synclinal lines, which have brought a lower and un- 
productive portion to the surface, and thrown the productive por- 
tions into a number of irregular troughs, ranging in the direction of 
the strike, east-north-east. 
The lower and unproductive coal-field is composed in part of 
coarse grits, well exposed on the banks of the Ruhr, between Her- 
