140 NEW YOEK STATE MUSEUM 



such dykes cutting across or through it for a long distance, and 

 to an unknown depth. These represent cracks or clefts by which 

 the country has been riven and which have been filled by the rise 

 of melted matter from below. They are all sizes, from half an 

 inch to 100 feet or more in thickness. 



Plutonic diykes aire not confined to Archaean regions. Dykes 

 of granite are seen in many places on New York island, penetrat- 

 ing in every direction the Lower Silurian mica-schist which forms 

 the masses of its territory. 



These are examples of a phenomenon frequently observed, viz. : 

 a plutonic rock penetrating strata of Paleozoic or later age. 

 They are similar in their origin to the out-flows of lava from 

 volcanoes. 



A prominent example of a late plutonic intrusion is seen in 

 the 'palisades ' of the Hudson, which is described under the Tri- 

 assic rock®. 



The plutonic and metamorphic rocks generally decompose 

 slowly and produce a poor or barren soil. The districts formed of 

 these rocks are the least fertile in our state, except where over- 

 lying deposits of glacial drift and alluvium furnish a soil which 

 is adapted to tillage and the support of vegetation. 



Typical Localities of the Archaean 

 The most southern locality of Archaean rock in New York 

 state is, on New York island, between 7th and 8th avenues 

 south of 155th street. This is a good exposure and is typical 

 of the Archaean gneiss of southeastern New York. This gneiss 

 is well shown throughout Westchester icounty along the shore of 

 the Hudson, though at a few points Lower Silurian limestone 

 and mica-schist occur. A little north of Peekskill may be seen 

 the granite mountains of the Highlands, which traverse Orange 

 and Putnam counties. These are chiefly massive, though on 

 their flanks are some gneissoid rocks and in many of the valleys 

 are Palaeozoic limestones and schists. Other localities are seen 

 in Dover mountain and in iStissing mountain in Dutchess county. 

 North of this southeastern area, the Archaean rocks are chiefly 

 confined to the region known as the Adirondack wilderness. 



