ITS 11. r. (^T'sinxG — aititte-nyicxtte (ixetss xkaii loox latck 



of them would seem to be justified in advance of a thoroiioh investio-a- 

 tion of their field relations. These rocks are widely and quite typicall}'' 

 exposed in the vicinity of Loon lake, in Franklin county, New York, 

 and many of the exposures are easily accessible; hence their selection 

 for descriptive purposes. The rocks are referred to the augite-syenites. 



Mkg ^scoprc Character 



When fresh these rocks are of a grayish green color, which quickly 

 changes to a more pronounced green on slight exposure. When longer 

 exposed a further change to a yellowish or In-ownish green takes ])lace, 

 and then a passage into a rusty brown, which is the prevailing color of 

 the exposures, except in recent cuts. The cause of these rapid color 

 changes is not manifest, sections from specimens of the first three shades 

 showing all the constituents to 1)e perfectly fresh. Even the rusty brown 

 rocks are often quite unaltered, though the hypersthene is commonly 

 decomposed, suggesting staining from the oxidation of the ferrous oxide 

 as a possible cause of the color here. 



For the most part the rocks are of medium grain, though with much 

 variation from place to place. They have been subjected to regional 

 metamorphism in common with the other rocks of the district, and 

 hence are rather evenl}^ granular, though in most cases larger crystals are 

 more or less abundant, suggesting cataclastic structure. Feldspar is 

 much the most prominent mineral, constituting usually about <S0 ])er 

 cent of the rock. The uncrushed crystals are always less green than the 

 granular feldspar. Pyroxene or liornblende and quartz make up the 

 rest of the rock. Sometimes a little garnet and magnetite may be dis- 

 cerned. Biotite is locally present. 



As a result of metamorphism, a rude foliation is commonly apparent, 

 though with much variation from place to place, depending on the 

 amount of ferromagnesian silicates present and on the degree of gran- 

 ulation. In other words, precisely the same range is shown that the 

 gabbros exhibit, though very coarsely crystalline phases comparable to 

 the coarse anorthosites have not been observed. In the more quartzose 

 varieties the ferromagnesian silicates retreat with disappearance of the 

 foliation, its place being taken by a rude linear structure due to the 

 drawing out of the quartz into pencils. 



Section near Loon IjAKe and its Interpretation 



In the east end of Loon Lake mountain, south of and within 2 miles 

 of the depot, the Adirondack and Saint Lawrence railroad has made three 



