Yol. 55.] TOESION-STEtrCTUEE OE THE DOLOMITES. 599 



Sass. As a matter of fact, the rock at the Dorthern end of the 

 terrace is called Cra di Mont in the district, and that of the southern 

 end is called Col di Stein, while the terrace is variously spoken 

 of as Pian de Sass or as ' the Lago,' from the presence of a small 

 tarn on it called Lago di Boe. 



The torsion of the Pordoi overthrust at Bova Alp. — 

 To return now to the circuit of Sella, the only means of tracing the 

 Pordoi overthrust-fault eastward is by following the continuation 

 of the different strike-and-dip systems of the Pordoi and the Sasso 

 Pitschi fault-blocks, respectively above and below the shear-plane of 

 the Pordoi Pass. In this way the fault may be traced in an east- 

 north-easterly direction as far as the Bova-Alp corner of Sella, 

 where the dolomite-rocks curve sharply northward. The curvature 

 is very unexpected, since the strike of the Pordoi series is almost 

 eastward and the dip slightly outward. 



The sharp curvature takes place where the Pordoi overfold is 

 intersected by a diagonal torsion-fault directed north-north-east and 

 south-south-west, parallel with a diagonal backle that runs through 

 the eastern side of the mountain. The overfold is broken up into 

 several shear-slices on that side.^ The diagonal fault crosses the 

 Cordevole Valley towards Col di Luc, but is subdivided into several 

 branches where it enters Sella. The north-north-east and south- 

 south-west branch passes through the mountain to the Groden Pass ; 

 its downthrow is westerly. A north-north-west and south-south- 

 east branch crosses the Pissadoi Eavine towards Ruon (p. 571) ; 

 its downthrow also is westerly. 



Strata below the shear-plane (east side). — TheAVengen 

 Cassian Series on the upthrow or east side of the Bova-Alp diagonal 

 fault is thus raised high on the Bova Alp relatively to the Schlern 

 Dolomite of the mountain. The Wengen strata are the typical 

 tufaceous shales and succeeding calcareous horizons ; the Cassian 

 strata include the fossiliferous marls with interbedded banks of Cipit 

 Limestone, and the high horizons of thick-bedded yellow limestones 

 crammed with echinoderm-remains. The average reading of the 

 strike in these strata between the Bovaiilp and the Campolungo Pass 

 is north 30^, 35° east, dip 40° west, which would indicate that the 

 Wengen-Cassian Series here is the continuation of the group of 



^ This is a case in point where a diagonal faxilt (which may be said to be 

 transverse, as it cuts an overfold across its strike) cannot therefore be said to 

 have originated subsequently. The phenomena of shearing differ on the oppo- 

 site sides, hence the probability is that here is an example of contemporaneous 

 faulting and shearing. I have treated this and similar cases in the Buchen- 

 stein Valley and the Grroden Pass as evidence of crust-torsion. The diagonal 

 fault and the overthrust have both crossed obliquely an older east-and-west fold. 

 I wish to emphasize this strongly, as in Mpine literature the great transverse 

 (diagonal) faults of the Alps are considered to be in the main a set of phenomena 

 subsequent to the thrusting in the Alps. But the transverse faults cannot be 

 said to be of later origin, unless it can be proved that the thrust-phenomena 

 on both sides of the fault are identical. In the Sella area virgating bundles 

 of faults occur towards any geographical direction ; some members of a bundle 

 may be shear-faults, while others may be vertical, but there is every reason 

 to consider them the result of one set of torsion crust-movements. 



