THE ADIRONDACK MOUNTAINS 39 



granite, because these rocks have been much more resistant to 

 weathering and erosion than the ordinary Grenville strata which 

 latter, therefore, generally occupy the valleys. 



The granite is a plutonic igneous rock which consists essentially 

 of quartz and feldspar (orthoclase), but which usually also con- 

 tains more or less black mica (biotite), amphibole, pyroxene and 

 magnetite. The syenite is similar, except that quartz is either 

 absent or much less prominent. Mostly, if not always, these two 

 rocks are merely variations of the great molten masses formed 

 during the cooling and crystallization of the rock. Accordingly it 

 is a very common thing to observe granite and syenite grading 

 back and forth into each other often within short distances, instead 

 of one cutting through the other as would be the case if one were 

 younger than the other. The typical syenite and granite are medium 

 grained, that is, they are made up of mineral crystals a few milH- 

 meters across. Locally the rock may be much coarser grained with 

 feldspar and quartz crystals up to an inch or more long. At times 

 some of the feldspar crystals are decidedly larger than the rest 

 of the minerals, giving rise to rock types known as granite or syenite 

 porphyry. Syenite is nearly always richer in dark minerals — black 

 mica, amphibole, pyroxene and magnetite — than the granite. Some- 

 times these dark minerals make up as much as half of the rock. 

 When freshly broken the typical syenite has a greenish gray color, 

 while the weathered surfaces are light to dark brown due to the 

 fact that the iron of the dark minerals has oxidized (weathered 

 out) to produce the brown stain of iron rust. This weathered 

 portion is seldom more than a few inches thick. Sometimes in the 

 woods on mountain sides, the immediate surface of the syenite is 

 nearly white with a brown layer just below the surface. The 

 explanation is that water charged with decomposing organic matter 

 has dissolved off the iron rust. The freshly broken granite varies 

 from greenish gray to light gray to pinkish or even reddish, the 

 color depending largely upon the color of the feldspar. Dark 

 colors are rare in the granite because of the relative scarcity of 

 dark minerals in the rock. 



Except for the gradations above referred to, the syenite and 

 granite are fairly homogeneous in large bodies, there being no 

 sharply defined layers as in the Grenville strata. One feature is, 

 however, important to consider, namely, the wavy or streaked 

 appearance presented by so many of the rock ledges of syenite and 

 granite. A close inspection reveals the fact that most of the 

 minerals have been flattened or drawn out into a sort of crude 



