I 



QUARRY MATERIALS OF NEW YORK 65 



porphyritic mineral in the midst of the finer material. This is 

 particularly observed in the metamorphic products of feldspathic 

 rocks like granite and syenite which often show lenticular remnants 

 of the original porphyritic feldspars and are known as " augen " 

 gneisses. 



Gneisses have all the variations in composition that are found 

 in the igneous rocks. Those of granitic composition are naturally 

 the most important for quarry purposes. Many of the granite 

 masses show gneissic phases on their borders, as is the case also of 

 the syenites, gabbros etc., the parallel lamination arising from 

 differential compression during consolidation or later. In some 

 places gneisses are formed by the injection of igneous material into 

 a hornblende or mica schist that is itself a modified sediment. 

 There are many such occurrences in the Adirondacks in localities 

 where the Grenville schists have been invaded by granite ; the latter 

 apparently in its cooling has given ofif solutions charged with 

 mineral materials which penetrated into the schist for long distances 

 and converted it into a firm, hard gneiss. The so-called granite from 

 Horicon is really an injected mica schist, with porphyritic feldspars 

 and quartz derived from igneous sources. The Manhattan schist 

 and Fordham gneiss as represented in most of the quarry localities 

 contain a large proportion of granitic material interleaving or com- 

 mingled with the ingredients from sedimentary sources, and it is 

 by reason of this injection that they are serviceable quarry stones. 



SERPENTINE 



The mineral serpentine is formed almost entirely by alteration 

 of other ferro-magnesian silicates, chiefly pyroxene and olivine. The 

 latter minerals, as has been noted already, are important constitu- 

 ents of the basic igneous rocks of the gabbro family, some members 

 of which are made up wholly of them. Their alteration, which 

 is a process of hydration largely, with the separation of more or 

 less lime as calcite and of some of the iron as iron oxides, takes 

 place readily under atmospheric weathering and leads to the for- 

 mation of extensive bodies of rock serpentine that has some use 

 for architectural and decorative purposes. 



There are several areas of serpentine in southeastern New York, 

 of which the largest is on Staten Island, covering all the higher 

 central part of that island. Other bodies are found on Manhattan 

 island (now concealed), at New Rochelle and Rye. The rock in 

 these places has little economic importance, owing to its badly 



