398 MR. A. V. JENNINGS ON THE [Aug. 1 899, 



to the serpentine is not yet worked out. In the Arosa area there 

 is a considerable development of rocks of the diabase-group, mostly, 

 according to Herr Bodmer-Beder,^ containing olivine : and the 

 diorites, spilites, and aphanites of Theobald and others occur along 

 the same horizon. The transition of the fine-grained types into 

 ' Griine Schiefer ' was long ago observed by Theobald ; Steinmann 

 has recently again called attention to it and shown that many 

 specimens of the ' Griine Schiefer ' are proved by their weathered 

 surfaces to be variolitic in structure. The presence of red layers, 

 and often hornstone and jasper, was also noticed by TheolDald, 

 and the rocks pass into ' mixed ' schists like those of the Parseun 

 slopes near Davos. The variolites are doubtless offshoots of some 

 of the igneous masses, and, I am inclined to think, of the less 

 basic types. In the Davos area, where the diabase series is 

 scarcely represented,'"^ I have not yet seen any variolitic structure in 

 the ' schiefer.' 



(vii) The * Talc-granite.' 



The frequent occurrence near the horizon of the Verrucano of a 

 white granite passing in places into an ' aplite ' or ' quartzite ' was 

 recognized by Theobald in Graubiinden and has been observed by 

 Heim farther west. It was termed ' Talc-granit ' or ' Talc-quarzit ' 

 by the former, but the white flaky constituent is, in many cases at 

 least, muscovite ; tourmaline also is frequently present. The age 

 and exact relationships of the rock still remain to be worked out. 

 In the Davos district it is conspicuous on the Mittelgrat, but I can- 

 not say whether it here lies partly iu the older crystallines or only in 

 the Casanna Schiefer : in any case, it occurs immediately above the 

 serpentine. On the Parsenn slopes it is found in fragments in the 

 crystalline breccia, and near the Schwarzsee Alp appears below the 

 serpentine and in contact with Lower Triassic limestone which is 

 altered by it. East of the Lareterthal it is found in contact with 

 and altering the red shales, and occurs again above Monbiel with Ver- 

 rucano and radiolarian chert. It thus seems to be intrusive, and of 

 later date than the Lower Trias, but its relation to the serpentine 

 I have not been able to make out. Whatever this may be, it is of 

 importance to note the well-recognized fact of the occurrence of the 

 talc-granite always approximately on the horizon of the Verrucano, 

 and its appearance here in association with the red shales and 

 radiolarian cherts. 



^ ' Ueber Olivindiabase,' etc., Neues Jahrb. Beilage-Band xii (1898) p. 247. 



^ The djke shown on Theobald's map near Drusatoha is, if present, not in 

 situ ; and the same may be the case with that at Selfranga. A small outcrop on 

 the Parsenn, near the Schwarzseealp, is, according to Dr. Ball, a fine-grained 

 enstatite-gabbro. A rock found at the foot of the Cotschna cliflF, in association 

 with the marble with serpentine-granules referred to on p. 397, is of a more 

 acid character. (The slice shows some grains of quartz, eaten into by magma, 

 roundish felspars, rather crushed and decomposed, without definite character, 

 and some rather corroded biotite: all in a matrix retaining traces of fluxion- 

 structure, but with indications of pressure and much micromineralogical change. 

 Probably once a rhyolite. — T. G. B.) 



