STAGES OF METATMORPHISM 425 



followed progressively by their crystalline alteration, with development 

 of certain new minerals — biotite, albite and staurolite. 



Then ensued the general impregnation of all the layers with pegmatitic 

 material, mainly quartz, orthoclase, oligoclase, and muscovite in num- 

 berless " augen," lenses, and parallel seams a few inches in thickness, 

 which often make up one-quarter to one-third of the present constitution 

 of the gneiss ; more or less apatite, pyrrhotite and black tourmaline also 

 become apparent in these seams. These two stages marked the progress 

 of static metamorphism. 



Next came the intrusion of a series of pegmatite dikes, cutting each 

 other in succession, and. all, so far as yet known, intersecting the pegma- 

 tite lenses of the preceding generation. With these orogenic movements 

 seem to have been connected, with extensive folding, crumpling, and 

 faulting of all the beds of gneiss, schist and limestone, a further increase 

 of crystalline structure, and development from micas and feldspars of 

 another group of minerals requiring conditions of high temperature, 

 such as muscovite, sillimanite, fibrolite, cyanite, and tourmaline. The 

 amount of shearing thus produced during dynamic metamorphism 

 varied in difFferent parts, being limited in degree toward the southern 

 end of the island, there apparently reaching only to a further stretching 

 and thinning of the layers, resulting in several systems of joints and 

 a slaty foliation, generally coincident with and rarely obliterating the 

 original bedding structure. An index of this limitation in the latest 

 period is shown in the pegmatite veins and dike intrusions, many of 

 which (probably of the earlier series) show more or less crushing, even 

 to the condition of schistose aplite, with development of cross-cleavage 

 and abundance of mica, but rarely to separation and isolation as lenses 

 rolled out within the schists. 



Finally, down to the present period, ensued the oxidation, hydration, 

 and partial leaching out of the less stable constituents of the schists by 

 meteoric waters, with partial decomposition of the amphibole into 

 hydrous forms of tremolite and asbestus, serpentine, ophicalcite, talc, 

 and chlorite; of the feldspar and micas into chlorite and margarodite; 

 of the feldspars into iron oxides, kaolin and zeolites, and of the pyrites 

 into ferric hydrate, alums, and sulphur. All these products have been 

 recognized on Manhattan island. 



Constitution of the Micaceous Gneiss 



In regard to the micaceous gneiss in the series previously referred 

 to (page 423), a detailed description has been published* on a specimen 



* J. p. Iddings : Bull. U. S. Geol. Survey, no. loO, 1898, p. 332. 



