64 NEW YORK STATE MUSEUM 



the polished Palisades stone in the State Museum. Ordinarily it 

 has no rift or grain and hence is difficult to reduce into dimension 

 blocks ; in some quarries, however, the stone splits readily enough 

 to be converted into Belgian blocks. 



The main area of diabase in this State is the Palisades intrusion, 

 a long north-south sill or sheet lying within shales and sandstones 

 of Triassic age and extending from Haverstraw to near Richmond 

 on Staten Island. The sill is from 300 to 800 feet thick. Its ex- 

 posed eastern edge with its vertical joint systems, forms the pre- 

 cipitous cliffs of the Palisades. This area has been a prolific 

 source of crushed stone which has been used in road-making and 

 concrete throughout the lower Hudson valley. There are countless 

 numbers of diabase dikes in the Adirondacks, particularly in the 

 northern and eastern sections, but they are mostly small, averaging 

 only a foot or two thick, occasionally reaching 20 or 30 feet, and 

 in one instance at Little Falls, nearly 100 feet. 



GNEISS AND SCHIST 



Gneiss and schist are general terms applied to the' metamorphic 

 silicate rocks whose original char'acters of texture, structure and, 

 not infrequently, mineral composition have been more or less com- 

 pletely changed under influences of compression, heat and chemical 

 agencies. Their chief structural peculiarity arises from a parallel 

 arrangement of the minerals, the light and dark components being 

 segregated in lines or bands which simulate the bedded structure of 

 sedimentary rocks. The planes of segregation, as in the case of 

 bedded struc'ture, mark the directions of actual or potential parting ; 

 the schists, particularly, have a very well-developed capacity for 

 splitting which resembles slaty cleavage in its perfection. 



The gneisses of more massive type are' suitable for general con- 

 struction purposes but ordinarily do not lend themselves to deco- 

 rative uses on account of their lack of uniform texture and appear- 

 ance, both of which vary with the direction of view. Such kinds 

 are mainly derived from granite and other massive igneous rocks. 

 Under the influence of powerful compressive forces, the originals 

 have been squeezed and stretched, bringing the scaly and elongated 

 minerals into parallel alignment and crushing the rest into granular 

 aggregates. The change may be not altogether a physical one, but 

 is generally accompanied by the development of new minerals and 

 more or less recrystallization of the mass. If the massive rocks 

 originally had a coarse or porphyritic appearance, very often there 

 will remain shattered but still distinct crystal aggregates of the 



