28 NEW YORK STATE MUSEUM 



Megascopic features 



The gabbro and its derivatives present a truly remarkable number 

 of facies or varieties clearly visible to the naked eye. The coarse- 

 ness of grain varies from the merest fraction of a millimeter to fully 

 an inch (for example, the stock on the south side of Loon lake). 

 The fine-grained portions are confined to the borders of the stocks 

 or the few branching tongues and were caused by the more rapid 

 chilling of the rock in those positions. Even the finest grained rocks, 

 however, are holocrystalline. As a rule the coarseness of grain in- 

 creases toward the interior of the masses, though often medium to 

 coarse grained rocks extend to the very contact. The typical or 

 prevailing gabbro shows a medium grain ; that is, the grains are 

 from I to 5 millimeters across. 



The texture varies from coarse to medium to fine-grained gran- 

 itoid, to medium to coarse-grained diabasic (ophitic). The gabbro 

 from the stock on the south side of Loon lake is an excellent example 

 of diabase texture in which the feldspar laths attain a length of an 

 inch or more. The typical gabbro always exhibits the diabasic 

 texture. 



In color, the gabbro and its derivatives range from nearly black 

 through dark to light gray, the darker varieties often showing a 

 slight reddish tinge due to the presence of garnets. The gray rocks 

 all belong to the more acidic (dioritic and syenitic) facies described 

 below. In one case a greenish gray color was noted. The very dark 

 color of the typical gabbro is due to the fact that the feldspars are 

 so charged with tiny black inclusions. 



In the typical gabbros the minerals commonly recognizable with 

 the naked eye or hand lens are plagioclase, pyroxene, hornblende, 

 garnet, biotite, and ilmenite, while in addition to these orthoclase 

 and quartz may often be seen in the more acidic phases. 



Except for the rather common presence of highly gneissoid to 

 even schistose amphibolite borders, the stocks of typical gabbro are 

 practically devoid of gneissoid structure. Some of the lighter 

 colored, more acidic phases, however, show fairly well-developed 

 foliation. 



It is important to note that many of the above described varia- 

 tions may be found within a single stock as, for example, south of 

 Mountain Spring lake. The following statements from Smyth's 

 description of a similar western Adirondack gabbro ^ fittingly apply 

 here : '* These [primary] changes in character take place very sud- 

 denly, and the different phases are most irregularly distributed, 



^Amcr. Jour. Sci., April 1896, p. 273-74. 



