44 ANNALS NEW YORK ACADEMY OF SCIENCES 



and enstatite are present as stated under pyroxenite. Small amounts of 

 olivine may be present, but if at all important the rock is classified as 

 olivine pyroxenite. Calcite, quartz, chlorite, antigorite and hornblende 

 may occur as alteration products; and in one more acid variety, contain- 

 ing some feldspar, zoisite was found, typically developed. The horn- 

 blende is the essential constituent in this class, making up from one third 

 to two thirds of the rock, and yet it seems to differ in the various patches. 

 It is usually of the basaltic variety, rich brown in color, and pleochroic 

 from dark to light brown. The refringence and birefringence are higher 

 than in the green variety, and the extinction angle is smaller. It is this 

 variety that usually gives rise to the poikilitic structure; it seems gen- 

 erally to have crystallized last. In one specimen, the hornblende was 

 apparently basaltic but had become a pale brown, and the crystals were 

 often decolorized in the center. Again it may be of the coarse greenish 

 brown variety which is characteristic of the hornblende norites, so that the 

 hornblende pyroxenites do not seem to be a very well defined member of 

 the series. 



Olivine Pyroxenite 



Olivine pyroxenite forms the last class of any importance; rocks in 

 which the olivine runs over one third and which are then classed as perido- 

 tites, are rare. They seem to occur, moreover, in the centers of the areas 

 of olivine pyroxenite ; and since they merely mark the culmination of the 

 segregational process which has led to the formation of these areas, it was 

 thought unnecessary to differentiate them on the map. As stated above, 

 the areas occupied by the chrysolitic rocks are rather vaguely defined, and 

 owing to the difficulty of distinguishing these rocks in the field, their 

 extent on the map is only as close an approximation as could be attained 

 by collecting and sectioning a large number of specimens. 



The weathering of the olivine rocks is beautifully shown in the two 

 eastern areas. The decay of the olivine causes a disintegration of the rock 

 into a coarse red sand, whose fragments are the grains of augite and 

 hypersthene. This is much prized as road metal, and it is used exten- 

 sively through the eastern part of the district. It is in effect a fine, re- 

 sistant, homogeneous gravel, found ready crushed and sifted, and is excel- 

 lent as a road covering. It is not, however, of like benefit to the farmer, 

 being too ferriferous to be fertile. On the hills, it favors especially a 

 peculiar flora, with an abundance of such trees as cedar and hemlock. 

 The topography of this region is rather striking, there being numerous 

 rounded hillocks, whose shapes are due to the melting away of the rock 

 masses which compose them. These ledges when artificially exposed show 



