MANHA TTAN ISLAND ROCKS AND MINERALS 9 



condition by heat and pressure combined with motion. The 

 crystalline limestone, which is almost a marble in some beds, is 

 much older, geologically speaking. lis present condition is due 

 to the severe metamorphism which it has undergone in company 

 with the schists. The hornblende schists are considered to be 

 of igneous origin, which would indicate that this region ex- 

 perienced volcanic activity millions of years ago. These schists 

 occur in comparatively small detached areas and are the highly 

 metamorphosed representatives of the molten lava, which was 

 forced into cracks in the old sedimentaries and solidified in the 

 form of trap dikes or was poured out upon the ancient surface in 

 sheets. The hornblende schist may be distinguished from the 

 inclosing mica schist by its darker color, due to the abundance 

 of black hornblende. Actinolite and tremolite schists occur in 

 company with some of the hornblende schists. East of the 

 Harlem River and north of Spuyten Duyvil Creek there is a 

 gneiss resembling some of the Manhattan Island gneiss in ap- 

 pearance, which is older than that and antedates even the mag- 

 nesian limestone. 



The minerals of Manhattan Island are, for the most part, 

 silicates of complex chemical composition, and they vary with 

 the nature of the rock with which they are associated. It is 

 comparatively easy, therefore, to group them according to their 

 occurrence into the minerals of (a) the gneisses and mica schists, 

 (6) the hornblende schists, (c) the granite and pegmatite veins 

 and dikes, {d) the limestone beds and {e) the serpentine area, 

 and this subdivision has been followed in the arrangement of the 

 collection. In connection with the study of the specimens the 

 visitor will do well to consult the geological map of New York 

 and vicinity attached to the case and the chart showing the 

 distribution of mineral localities on Manhattan Island which has 

 been placed in the case. The chart is reproduced herewith. 



The minerals of the gneisses and schists are primarily quartz, 

 feldspar and mica, the last predominating. In this association 

 these minerals rarely occur upon Manhattan Island as handsome 

 cabinet specimens, but there occur with them, as secondary 

 effects of the metamorphism of the original beds, fibrolite, 

 epidote and other species. Crevices in the schists have given 



