100 NEW YORK STATE MUSEUM 



feldspar. Similar rocks are extensively exposed in the eastern) 

 Adirondack^. They are at the surface over a large part of Essex 

 county, and it has long been known that they extend into Frank- 

 lin, an attempt to indicate their probable limits being made by 

 Merrill in 1894. 1 One of the surprises of the recent field work in 

 the county has been the very considerable area occupied by them,, 

 they running considerably beyond the limits assigned to them by 

 Merrill from the very limited information at his command. 



The anorthosites in the main are coarse to very coarse grained- 

 rocks consisting almost wholly of labradorite feldspar, whose 

 large blue black crystals frequently reach a length of 3 inches 

 and occasionally are a foot long or more. Not infrequently the 

 crystals show the iridescent play of colors so often seen in this* 

 mineral, but nothing of the sort has been noted which will for a 

 moment compare with the Labrador or Norway material. The 

 extinction angles of the feldspar show that it is mostly labradorite 

 though occasional angles of 30° and over indicate that more basic 

 feldspars are also present in slight amounts. As accessory min- 

 erals ilmenite, apatite, titanite, garnet, augite, hypersthene, horn- 

 blende, biotite, pyrite, pyrrhotite, chal copy rite and a little ortho- 

 clase and quartz have been noted, the two latter occurring only- 

 in minute patches in the sort of intimate intergrowth known as 

 granophyr. Ilmenite, augite, hornblende and one of the three- 

 sulfids are the usual accompanying minerals, the others are only- 

 occasional and all are in slight quantity. 



The structure is cataelastic, that is the rock has been subjected 1 

 to great stresses, while deeply buried beneath other rocks since- 

 worn away, which have had the effect of more or less completely 

 breaking down the large crystals into a multitude of small frag- 

 ments, while at the same time the great pressure of the overlying" 

 rocks kept the rock at all times in an unyielding condition, instead 

 of permitting it to be crushed to loose fragments. All grades of 

 this occur from very coarsely crystalline varieties, with only a 

 little granular material around the edges of the crystals, to those 

 in which the granulation is quite complete, few or no large 



lMerrill, F. J. H. Economic and geologic map of the state of New York. 



