42 NEW YORK STATE MUSEUM 



the whole problem of this erosion and its significance is discussed 

 a few pages beyond. 



At some time after the vast bodies of syenite-granite were 

 thoroughly consolidated, but certainly before the Paleozoic era, 

 there were several relatively minor intrusions of molten rock. These 

 times of intrusion are very distinctly separable and the rocks are of 

 quite different appearance. First of these was the so-called gabbro. 

 This rock varies considerably though it is always dark gray to 

 black and usually medium to coarse grained ; that is, the mineral 

 constituents range from one or two millimeters to fully an inch 

 in length. Typically the rock contains feldspar (plagioclase), 

 pyroxene, hornblende, black mica (biotite) and magnetite with 

 various other minerals in small amounts. Sometimes the gabbro is 

 difficult to distinguish from certain of the dark border phases of the 

 anorthosite already described, though usually the long, lath-shaped 

 feldspar crystals irregularly inclosing the pyroxene or hornblende 

 characterize the gabbro. Also the mode of occurrence of the gabbro 

 is different from that of the older intrusives. Instead of breaking 

 up, raising up or tilting the country rock, the molten gabbro appears 

 to have penetrated the earth's crust through practically vertical, 

 cylindrical or pipelike openings seldom more than one or two miles 

 in diameter. As seen in the field, the gabbro is a typical plutonic rock 

 with sharp, practically vertical contacts against the older (country) 

 rocks. Sometimes the gabbro' exhibits a streaked appearance 

 (gneissoid structure) due to a crudely parallel arrangement of 

 minerals resulting from movements or currents in the still partially 

 molten masses under pressure. These bodies of gabbro are scattered 

 throughout the Adirondack mountains and are well exhibited on 

 most of the published geologic maps (see appendix), fully sixty 

 occurring within the North Greek quadrangle alone. 



Another interesting igneous rock, and still later than the gabbro, is 

 the so-called pegmatite which is really a sort of very coarse- 

 grained whitish granite. It consists almost wholly of quartz and 

 feldspar in crystals from one-half to several inches across. Usually 

 it contains more or less white mica (muscovite). This rock occurs 

 in the form of dikes, that is, it fills fissures from an inch or two 

 wide and a few yards or rods long to 50 or 75 feet wide and many 

 rods long. Such pegmatite dikes may be seen cutting through all 

 the previously described rocks with very sharp contacts, and hence 

 they are clearly younger. Pegmatite dikes are very common 

 throughout the Adirondacks and they are easily recognized. 



