226 ANNALS NEW YORK ACADEMY OF SCIENCES 



pyrite. This rock was converted into hornblende schist by mechanical 

 deformation accompanied by molecular rearrangement of the augite and 

 feldspar. The changes which resulted in the formation of the hornblende 

 schist of southeastern New York were probably very similar to those 

 which have occurred in the case of the Scourie dikes. 



The lenticular to tabular shaped masses of epidosite occasionally ob- 

 served in the hornblende schist associated with small stringers of pegma- 

 titic material represent an alteration which has occurred since the devel- 

 opment of the foliated structure, since the remnants of unaltered horn- 

 blende in the epidosite show the same parallel alignment as those in the 

 normal schist. The hornblende and feldspar of the original hornblende 

 schist along these zones have been converted into epidote. This was 

 brought about by some marked changes in chemical composition, as a 

 comparison of an analysis of the hornblende schist with one of the epido- 

 site developed from it will show. Such analyses are given in a previous 

 paragraph. The change was accompanied by a partial oxidation of the 

 iron and a very noticeable reduction in the percentage of magnesia, it 

 being less than one-half as high as it is in the hornblende schist, with a 

 correspondingly large increase in the amount of lime present. The alka- 

 lies almost disappeared during the alteration, while the percentages of 

 the other constituents remained practically the same. 



Dr. Julieii,^* in his study of this phase of alteration in the hornblende 

 schist, came to the conclusion that intense local compression and strain 

 were necessary for its development and that it was not connected with the 

 process of change to pegmatite. This alteration does occur along minor 

 fracture zones in the hornblende schist, but where observed by the writer, 

 the injection of pegmatitic materials also accompanied those where alter- 

 ation to epidote has taken place. It seems more plausible, therefore, to 

 think that the circulation of the solutions which brought about the neces- 

 sary chemical changes involved in this alteration did accompany the peg- 

 matitic injections which occur in these fractured zones. 



The actinolite, tremolite and talc schists occasionally found interstrati- 

 fied with the mica schist, especially on Manhattan Island, are very similar 

 in their mode of occurrence to the hornblende schist just described. They 

 undoubtedly have a similar origin, in that they represent much metamor- 

 phosed intrusive sheets of basic igneous rocks. In composition, these in- 

 tnisives were probably somewhat more basic even than those from which 

 the hornblende schists have been derived, since in order to get a meta- 

 morphic rock made up largely of such minerals as actinolite, tremolite 

 and talc, it would be necessary to have an igneous rock high in magnesia 

 and lime and comparatively low in silica, alumina and the alkalies. 



" Op. cit., p. 446. 



