ROGERS, GEOLOGY OF THE CORTLAXDT SERIES 45 



their peculiar softened shape, with rounded masses often projecting high 

 above the general level of the rock. This is well shown at Chase Corners, 

 just northwest of Dickerson Hill, as pictured in Plate III, fig. 1 ; the 

 effect is rather striking, when it is recalled that the other rocks of the 

 series are usually found planed smooth by glacial action. 



As above stated, these rocks resemble in general aspect the pyroxenites, 

 although closer examination may reveal the small yellowish green grains 

 of olivine. It is only in thin section, however, that the rock may be ac- 

 curately studied. The proportions of augite (usually the gray variety, 

 but occasionally the pink), enstatite and Irypersthene vary as recorded 

 under the pyroxenites ; almost pure olivine-augite rocks and olivine- 

 hypersthenites are known. Hornblende, usually basaltic, is a fairly con- 

 stant component, though it seldom occurs in important amount. The 

 olivine is present in amounts varying from one-fifth to one-third of the 

 rock. It occurs in typical grains, usually rounded, but sometimes ex- 

 hibiting a distinct crystal outline. Usually besides these large grains 

 (which may reach 3 mm. in size), there are numerous little ones scattered 

 throughout the augite. The mineral is colorless but often carries magne- 

 tite inclusions in the form of minute rods ; this feature is, however, better 

 developed in the peridotites of Stony Point. The alteration is exhibited 

 in all stages. Sometimes the mineral is nearly all serpentinized, with 

 only occasional little bullets of olivine remaining, while again it may be 

 almost perfectly fresh, even when strongly cracked, and when the augite 

 shows distinct alteration. The characteristic mesh structure is always 

 developed on alteration. The product is, of course, serpentine. Antig- 

 orite is the most common variety, chrysotile is somewhat more rare and 

 bowlingite less frequent. Magnetite separates out in varying quantity. 

 According to Williams, whenever the mineral comes in contact with feld- 

 spar a diopside-actinolite reaction-rim is formed, such as he has figured ; 32 

 but feldspar has not been found in contact with the olivine in the rocks 

 of the main area, so that this effect is not a common one. A similar 

 phenomenon, between olivine and biotite, is described and figured below. 



Peridotite 



Peridotites are merely exceptionally basic phases of the rock last de- 

 scribed; and their relations to that type have already been discussed. 

 They do not differ in any respect except the proportion of olivine ; this 

 mineral, however, in several cases constituted about three fifths of the 

 rock, and in one instance was almost unaltered, though badly cracked. 



^Amer. Jour. Sci., (3), XXXI, 35. 1886. 



