Geological Society. 399 



intermediate between the carboniferous and Silurian systems. Should 

 such a group exist on the continent, then would the Devonian sy- 

 stem be established, not merely on plausible arguments derived from 

 its suite of fossils, but also on the more direct evidence of natural 

 sections. 



With these views the authors endeavoured (1.) to ascertain the 

 natural descending order of the formations on the right bank of 

 the Rhine, between the Westphalian coal-field and the chain of the 

 Taunus. (2.)- To ascertain the same order in Belgium, and among 

 the ancient rocks on the left bank of the Rhine, north of the Hunds- 

 ruck. . . 



In the course of the summer they also made (with smiilar 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 then- 

 paper. But before commencing their detailed sections, they explani 

 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 Avas 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 phaenomena of the Belgian 

 provinces*. The authoi-s, 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, S/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, 

 * See L. E. and D. Phil. Mag., vol. xvii. p. 303.— Edit. 



