QUARRY MATERIALS OF NEW YORK 6l 



stone serviceable for polished and decorative work. The deep 

 green of the Adirondack syenite is very characteristic. Anorthosite 

 is either gray or dark green, the latter being characteristic of the 

 feldspar in its original state, while gray is peculiar to the crushed 

 and recrystallized varieties. The uncrushed feldspar shows the 

 blue iridescence common to labradorite which adds much to the 

 beauty of polished samples. 



There are no peculiarities in the weathering of the two rocks, 

 and they yield the same decomposition products named under gran- 

 ite. On the whole, syenite appears more resistant to frost action 

 than the latter, at least it seldom breaks up into a granular aggre- 

 gate which not infrequently marks the outcrop of granite bodies. 

 As to the durability of anorthosite, little can be said from the point 

 of practical experience since it has not been used very long for 

 outdoor work. The rock, in place, shows little change on the 

 surface. At Augur lake, near Keeseville, there are vertical cliffs 

 of anorthosite which have been directly exposed to the weather 

 ever since the glacial period ; these show a bleached film not more 

 than one-fourth of an inch thick coating the surface, but no stain 

 or softening. This appears a favorable indication of its permanency 

 under atmospheric conditions. 



DIORITE 



The name diorite is used to denote a rock containing plagioclase 

 and hornblende as essential minerals. The plagioclase is nearer the 

 albite than the anorthite end of the series, including such varieties 

 as oligoclase and andesine ; the hornblende is the same kind that 

 accompanies syenite or granite and is usually plentiful. The color, 

 consequently, is rather dark, with the grayish tones predominating. 

 Some diorites contain considerable biotite which, if it gains ascend- 

 ency over the hornblende, makes a mica-diorite as distinguished from 

 the hornblende type which is simply a diorite. The composition of 

 the diorite is intermediate between that of granites on one side and 

 the gabbros on the other, and it might be expected to find gradation 

 toward either series, through the appearance of certain characteristic 

 minerals. The mingling of quartz and alkali feldspar makes a 

 rather common variation from the type, leading to the class of 

 granodiorites which may be described equally well as basic granites. 



The diorites are not common rocks in this State. There are no 

 large areas of typical massive diorite ; some of the gneisses in the 

 Adirondacks are related to diorites in mineral composition, having 

 perhaps originated from such rocks, though now changed to the 



