January 24, 1901] 



NATURE 



295 



(i) Partly to the fact that all these overthrust masses 

 and the basal mass in the district of the Glarus and 

 Rhatikon have been pushed subsequently towards the 

 north-west above the folds in the outer, or " molasse," 

 zone of the Alps ; thereby the original inclinations of 

 the several thrust-planes have been altered or masked. 

 (2) Partly to subsequent displacements effected along 

 longitudinal and transverse fault-lines. (3) Partly also to 

 the insufficient knowledge of the details of the strati- 

 graphical succession as exhibited in the different facies of 

 the Helvetic and the Austrian Alps. 



What occurs to me in reading these works is that a 

 great number of the observations which Prof. Rothpletz 

 has given are unaccounted for in the conception of the 

 structure as yet presented by that author. The concep- 

 tion does not go far enough, it misses the significance of 

 the author's carefully collected data of strike and dip 

 which prove intercrossing and curved strikes to be really 

 the leading structural feature of the district. Prof 

 Rothpletz does not offer any explanation of that remark- 

 able fault-curve in his map which follows the Linth 

 Valley N.N.E. as far as the Schild Mountain, then 

 curves east to the Sees Valley, and again S.S.E.' and 

 S.E. to Sargans ; or interpret that other zig-zagging fault 

 which curves round the south of the Rhatikon and south- 

 east through the Falknis Chain, then southward in the 

 Silvretta Massive, and again south-west through the 

 Lenzerhom Chain. According to my experience in the 

 Dolomites, and having regard to the variations of the 

 strike in the folds of the thrust-masses, I would consider 

 these as primarily strike- faults through curved folds that 

 formed round a local crust-basin {cf. Geological Magazine, 

 1894, pp. 54-58 ; Q.J.G.S., 1899 ; Geographical Journal, 

 1900). 



Let any one glance at my figure of the torsion-curves 

 round the northern periphery of the Adriatic crust-basin 

 (Z,.C, 1899, fig. 22). The curves are convex to the 

 north ; the chief overthrusts have come from the 

 W.N.W., N.W., N. and N.E., and have moved centri- 

 petally with reference to a centre in the Adriatic depres- 

 sion. Or compare the much smaller fold-arc of the 

 Grdden Pass arch and its accompanying fault-curve from 

 Plon over the Groden Pass to Ruon and Corvara. 



The arc of origin of the thrust is convex to the north ; 

 the thrust-mass has advanced southward and been broken 

 up into several fault-blocks by faults, for the most part, 

 transverse to the arc of origin. Return, now, to the map 

 of the Glarus (Rothpletz's Atlas, Taf xi.), and it will be 

 clear that there is a repetition of similar geological 

 phenomena. Even the details in the typical fold-forms 

 are essentially alike (compare Rothpletz, Plate v.. Figs. 

 5, 6, with sections 4, 5, 6 and 16, 17 in my "Torsion- 

 structure.") In passing, it may be said that this similarity 

 is in so far important, as it shows how much closer is 

 the resemblance in the structure of the Eastern and 

 Western Alps than has been usually supposed. 



My interpretation of Prof. Rothpletz's data may now 

 be indicated. His description of the continuation of the 

 Glarus thrust-mass westward and eastward from the 

 Miirtschen-Stock shows that it is broken by numerous 

 faults along which both horizontal and vertical dis- 

 placements have taken place, and that the typical fold- 

 NO. 1630, VOL. 63] 



form has undergone quite different modes and degrees 

 of compression in the several fault-blocks. From the 

 Firz Stock in the east to the Linth Valley in the west one 

 is presented with a series of uplifts and downthrows 

 having the character of incipient folds, where arches and 

 troughs are limited by steep septa or faults. The Miirt- 

 schen and Firz group is buckled into three main fault- 

 blocks, the central of which is the highest, while the 

 Schild group is depressed relatively to the eastern group, 

 but within itself shows also a central upthrow flanked by 

 downthrown blocks. The faults which have determined 

 this cross-buckling are practically transverse to the local 

 curvature of the strike. They therefore represent what I 

 termed " radial " faults, or the fault-radii of a fold-arc 

 {QJ.G.S., 1899, p. 604). This cross-blocking of a fold- 

 arc demonstrates the action of horizontal pressures 

 parallel with the direction of the curved strike, in ad- 

 dition to the action of horizontal pressures rectangularly 

 to the fold-arc, and indicates that we have here a system 

 of local pressures complete in itself. 



I further note from Prof. Rothpletz's sections that the 

 horizontal compression along N.W.-S.E. Hnes has been 

 much stronger in the case of the Schild group than in 

 that of the Mtirtschen group (i : Taf. vii., Figs. 8, 15). 

 The N.W.-S.E. strike represents the general Alpine 

 strike at this part of the Alps, and it will be observed that 

 in the Schild area the " Alpine " strike coincides, or is 

 very slightly oblique, with the local strike, hence the 

 resultant crust-deformation here represents the sum of 

 two almost similarly-acting sets of pressures. Moreover, 

 it is well known that the Alpine movements became 

 rapidly less intense from west to east in this boundary 

 district between the Western and Eastern Alps. The 

 predominance of the N.E.-S.W. or "Alpine" strike in 

 the basal mass of plastic strata, and the general over- 

 casting to N. and N.W. of the folds, both in the basal 

 mass and the thrust- mass, together with oblique over- 

 thrusting between the fault-blocks, seem to me conclusive 

 evidence that the "Alpine" movements were not only 

 subsequent to the formation of the fold-arcs, but also 

 subsequent to the initiation of the N.N.W.-S.S.E. 

 faults through the fold-arc {cf footnote, Q.J.G.S., 1899, 



P- 559)- 



Take, again, the Rhatikon thrust-mass in Prof. Roth- 

 pletz's map (2, p. 161). Faults virgate from Bludenz to 

 W.S.W. and S.W., and cut the curved fault which 

 separates the Rhatikon and the Falknis crust-blocks. 

 These faults I take to have been originally fault-radii of 

 a Rhatikon fold- arc formed round the north-east of the 

 crust-basin. The overthrust of that mass has taken place 

 in the main from this direction towards the south-west. 

 Each fault-block in this thrust-mass is gently folded 

 between the Montafon Valley and the Falknis, and a 

 N.W.-S.E. strike is present in the Gorvion and other 

 parts close to the Falknis Chain. This strike, together 

 with the folding, indicates the action of horizontal pres- 

 sures from the north-east ; and the virgating group of 

 faults across the Rhatikon indicates the action of hori- 

 zontal strains parallel with the curved strike of the fold- 

 arc, effecting transverse fractures in the sense of fault- 

 radii of the arc. Hence there is in the Rhatikon evidence 

 of a complete set of local phenomena of compression 



