January 31, 1895] 



NA TURE 



j'o 



shoun to be due to tV e thrusting of one set of beds on to 

 another, either horizo- tally or at a low angle. Rothpletz 

 shows that the;e overthcusts are even wider in their dis- 

 tribution than is now ueierally admitted. He describes 

 tiic most important ca^os, summarises the literature and 

 \ ariou; theories regarding them, and states the explana- 

 tion which seems to him to agree best with the facts. 

 Most ot the examples quoted, the author has personally 

 examined. 



The first case taken is that of the classical Linth in the 

 Glarus. This, as interpreted by Escher and Heim, has 

 exercised a great influence on geological thought. The 

 valley has previously been regarded as due to a great 

 double fold. Rothpletz, however, maintains that it is a 

 ■' graben " or rift-valley, formed by the subsidence of the 

 block of material which once filled it up. One difficulty 

 that hitherto told against this explanation, was the fact 

 that the marginal faults had never been discovered. Roth- 

 pletz, however, maintains that they are there ; he de- 

 scribes them at one point, in a section which, he declares 

 p. 10), "must silence the most utterly sceptical." The 

 denial of Heim's famous double fold necessitates a new 

 interpretation of other features in the geology of the 

 lountry. Thus the rocks in theSchild — the mountain to 

 the west of the town of Glarus — have been explained by 

 Heim as the crushed-out beds of the middle limb of the 

 fold. Rothpletz, however, maintains — and his evidence 

 seems conclusive — that they are due to an overthrust. 

 He adds a further difficulty to Heim's theory, by showing 

 that if true it is inadequate as it stands. The country is 

 more complex than a double fold can explain ; a treble 

 and a quadruple fold at least must be assumed, for the 

 beds repeat themselves more than thrice. This Roth- 

 pletz explains by the assumption of three overthrusts, 

 which he names after Schild, Kapf, and Plattenalp. 



The next case considered is that of the mountain mass 

 of Sentis, to the north of the Glarus area. This was 

 described by Escher von der Linth in 1857. It was then 

 said to be remarkable in having a great series of faults 

 crossing the axes of the folds, but none parallel to them. 

 This was confirmed by the maps and memoirs of Escher's 

 pupils, and Suess, therefore, in l8cS5, made the Sentis 

 thetypeof a class of mountain structure named" Blatter." 

 Some discrepancies between the descriptions and the 

 maps led Rothpletz to re-examine the country. The 

 result is that he finds numerous faults parallel to the 

 ridges, as well as across them, and also a series of over- 

 thrusts which occurred later than the folding, and earlier 

 than the transverse faults. 



From the Sentis it is natural to turn to the Juras. 

 This range has long been famous owing to the ingenious 

 devices designed to enable the fold theory to account 

 for the rock sequences that occur there. But the 

 "vanishing trick" diagrams, by which the absence of 

 certain beds has been explained, have always been 

 viewed with suspicion ; they seemed too much like the 

 schemes by which Ptolemaic astronomy was reconciled 

 with facts. Miiller in i860 demonstrated their insuffi- 

 ciency, but they still survive. Rothpletz discusses this 

 fold theory in its three most plausible modifications, viz. 

 the faulting of over-folds ; the folding of an area after the 

 rocks in the centre have been raised by a double fault ; 

 lateral contraction forcing one side of a valley of erosion 

 NO. 1318, VOL. 51I 



over on to the other. Rothpletz dismisses these, and 

 accepts the theory of overthrusts along slightly inclined 

 thrust-planes. 



The fifth case is that of the highlands of the north-west 

 of Scotland. This is so well known, that the author adds 

 nothing new, except a doubt as to the relation of the 

 minor and major thrusts. He notes with relish the 

 abandonment of the view at first announced that the 

 thrust-planes started as a result of the inversion of over- 

 folds. 



A simple example of overthrusting in a much later geo- 

 logical period is afforded by the granite of Lausitz in 

 Saxony, which occursabove the Turonian limestones. This 

 superposition was explained at first by the chalk having 

 been deposited under an overhanging cliff, and then by 

 the granite having been dropped as an erratic. Both 

 these theories were ridiculously inadequate. The most 

 popular explanation assigned an eruptive origin to 

 the granite ; the absence of contact-alterations and of 

 apophyses from the granite is fatal to this. Overthrust- 

 ing is the only theory left, and this Rothpletz accepts as 

 satisfactory. The earth-movements in the coal-fields of 

 Westphalia, Belgium, and northern France are next 

 considered : these have long been known to be ex- 

 tremely complex. The explanation now accepted, is 

 summarised with great lucidity, and illustrated by over 

 thirty figures and three plates of sections. It attributes 

 the present structure of the country to the inthrusts of 

 slices of complex composition between other deposits. 

 The last case considered is that of the area of the coast of 

 Provence and the French Alps. The author has not 

 personally examined the ground, and so would have pre- 

 ferred not to discuss it. Haug, however, has suggested 

 that the assumed " pli-failles" or fold-faults) are often 

 only inverse faults, and that the latter are capable of 

 explaining the phenomena without the hypothetical folds. 

 Rothpletz, therefore, thinks it advisable to discontinue 

 the use of the word " pli-faille," and suggests the substitu- 

 tion of " faille de recouvrement," or some other term 

 which does not beg the question. 



In the concluding chapter, Rothpletz summarises the 

 general characters of overthrusts. He remarks that their 

 importance is being more widely recognised, and that 

 they are accepted now in explanation of many phenomena 

 for which the agency of folding was formerly invoked. 

 He thinks that they probably always occur in mountain 

 formation. He discusses their relations to the earth- 

 movements with which they are associated, as far as our 

 present knowledge enables these to be generalised. Thus, 

 the thrust-planes occur approximately parallel to the 

 folds, but the inclination of the planes is usually in the 

 opposite direction to that of the mountains : this might 

 have been expected, as it is in harmony with some of 

 Daubree's experiments. Divergences between the strike 

 of folds and overthrusts, however, occur, and are ex- 

 plicable by the later origin of the latter. The inclination 

 of thrust-planes is almost always different from that of the 

 beds or folds ; the former, however, both above and below 

 the thrust-plane, occur in their normal sequence. Friction 

 breccias, mylonites, &c., occur along the thrust-plane, 

 while " schleppung," or terminal curvature of the beds 

 toward the plane, is generally developed. A single thrust- 

 plane may occur; but, as a rule, there are many parallel 



