1896.] NEW YORK ACADEMY OF SCIENCES. 25 



comprise a part of the central gneissic core of the mountain 

 range which is surrounded on either side by Siluro-Cambrian 

 strata and intrusions of various igneous rocks. The region has 

 undergone great orographic disturbances, as is instanced in 

 the overthrust b}' which the gneiss has been made to overlie 

 the later sedimentary rocks and in the general folding and 

 crumpling which is to be seen everywhere. Long continued 

 weathering and erosion have, however, removed all sharp out- 

 lines of the topography, and when any notable elevations occur, 

 they are to be accounted for by the greater resistance which the 

 rock has offered to these influences, as is seen in the eclogite 

 hills. The latter form a well marked topographic feature of this 

 part of the mountain range, extending along the easterly edge of 

 the gneiss near the contact of the latter with granites and dio- 

 rites, and then curving off to the west where they are in contact 

 with sediments and basfc eruptives. The lack of good exposures 

 necessarily renders the relations which these rocks bear to one 

 another difficult to ascertain. Where contacts of eclogite and 

 gniess were found they proved in most cases to be faulted ones 

 and could not be used to draw any conclusions as to interbedding 

 or intrusion. V. Giimbel states that the former occur as lense- 

 shaped inclusions in the latter and cites instances which seem to 

 favor such a view. Riess * also adoi)ts this conclusion and ar- 

 gues for a non-eruptive origin by sa^-ing that the eclogite stands 

 in close geological and mineralogical relations to hornblende- 

 gniess to which such an explanation could not be applied. 



But under the general term of hornblende-gneiss he includes 

 two diflerent types of rocks — a basic and an acidic. The latter 

 is geologically independent of the eclogite and forms a great por- 

 tion of the banded rocks of the region. The former is composed 

 of hornblende, garnet, a basic feldspar, and other less important 

 minerals. It always accompanies eclogite and grades into it, so 

 that no sharp lines can be drawn between them. Wherever con- 

 tacts of the two different types were found, they were always 

 sharp and well defined, with no signs of transition. Exposures 

 illustrating this occur at Stammbach, Fattigau and near 

 Wustuben. 



In general it may be said that the eclogite possesses a certain 

 degree of sameness over the whole area, but there is too much 

 mineralogical variation to place any definite limits on the term. 



The most characteristic mineral and one which always forms 

 a large percentage of the total mass is garnet. Its pale red to 

 dark red color in contrast with the bright green of the pyroxene 

 renders it easily distinguishable, even when the individuals are 



* MiD. u. Pet. Mitth. 1878, p. 240. 



