MISS M. M. OGILVIE [mES. GORDON] ON THE [Aug. 1 899, 



Evolute and involute torsion-movements at the 

 summit. — The Jurassic strata have subsided upon a sliding- 

 plane and have been twisted south-eastward in proportion as the 

 underlying Dachstein- rocks have been in their main body over- 

 thrust to the north-west towards the periphery, and in smaller 

 mass twisted south-eastward along with the Jurassic strata. At the 

 same time the Dachstein rocks of Yallon below the Eis-See summit 

 have been twisted south-westward, a movement precisely the converse 

 of that which has taken place in the higher rocks between the Boe 

 and Eis-See summits. This movement round the Eis-See summit 

 repeats the C-spiral movement characteristic of the south-eastern 

 quarter in the Groden-Pass scheme, while the movement in the 

 Jurassic infold and Boe ridge repeats the movement of the D-spiral 

 in the south-western quarter of the Groden-Pass scheme. 



The southern part of the summit-buckle is overthrust round a 

 small southern fold-arc parallel with the Pordoi and Bova-Alp 

 fold-arc in the southern periphery of the mountain. The underlay 

 of the Dachstein-rock has been tilted inward and sliced northward, 

 while the overlay has been thinned towards the south-west and 

 south-east. These complex movements repeat the A & B spiral 

 movements already demonstrated in the southern part of the peri- 

 pheral overthrust. (See fig. 19, p. 607.) 



The similar appearance presented by the curved masses of 

 calcareo-dolomitic strata at Sasso Pitschi and the southern part of the 

 Boe summit- ridge is therefore no chance resemblance, but a result 

 of precisely similar crust-torsion at the one place and at the other. 



The same spiral directions of twisting movement, therefore, 

 explain the position of the strata in the main mass of the mountain 

 and in the summit-area. Centrifugal forces have pushed outward 

 and upward the Dachstein horizons of the summit-buckle, while 

 centripetal forces have pressed inward and tilted downward the 

 Dachstein and Jurassic horizons of the summit-trough. Thus crust- 

 movements which may be termed ' evolute ' and ' involute ' have 

 taken place in reference to a central area upon which the Eis See 

 rests. The lake-area is that in which radiating faults converge, 

 and apparently denotes the superposition of a diagonal arch upon 

 an earlier east-and-west arch. 



Fan-structure. — The Dachstein buckle on the top of Sella 

 affords a miniature example of Alpine fan-structure. It rises from 

 the midst of a circumferential trough much elongated in shape. 

 The fundamental fact is that its position here is the result of 

 combined translatory and rotatory movements. The 

 older rocks have been twined above younger horizons. 



In the same way the peripheral overthrust-rocks of the Sella 

 Massive have been twined in different directions outward and 

 upward relatively to inward and downward flexures of younger 

 strata. These flexures represent the synclinal curves corresponding 

 to anticlinal curves round the mountain. Hence the mass of rocks 

 composing the overlay of the peripheral overthrust rises from a 

 circumferential trough proportionately larger than the trough below 



