THE BASIC MASSIVE ROCKS, ETC. 821 



As to the origin of the supposed tremolite there is a doubt. 

 The assumption of fibrosity by the diallage in some cases, would 

 seem to point to a secondary origin for the bundles of horn- 

 blende since this is the only fibrous mineral in the rock. The 

 little bundles, however, are so compact and their situation within 

 the thin section is so remote from that of the diallage that the 

 supposition of a primary origin for them seems to be demanded. 



The magnetite, which is much more abundant in these rocks 

 than in the normal gabbro, occurs as inclusions in all their con- 

 stituents. It is more commonly an attendant of the tremolite, 

 however, than of either the diallage or the olivine. It occurs as 

 small irregular grains between the fibres of the bundles and in the 

 exterior portions of the fibrous groups, and in nearly all cases 

 the longer directions of the grains are parallel to the long axes 

 of the fibres. 



In mineral composition these rocks are wehrlites, but their 

 structure is quite different from that of any rocks of this char- 

 acter heretofore described. The diallage and tremolite act as 

 interstitial substances inclosing olivine grains, where the first two 

 minerals are in sufficient quantity to serve this purpose. Where 

 they are absent the rock consists entirely of a mosaic of inter- 

 locking grains of perfectly fresh olivine. Since both the olivine 

 and the diallage are identical in characteristics with the same 

 minerals in the normal gabbro, we are led to suspect that the 

 rock is a special phase of the latter, in which the acid constituent 

 — plagioclase — is lacking. 



Pyroxene Aggregates. — In other beds the olivine is in less 

 quantity than in those just described, and the rock composing 

 them is slightly different. The olivine is in the same small 

 grains, but these no longer form mosaics. They are in greater 

 part included within large irregular plates of green pyroxene, 

 whose ragged edges extend out for some distance between other 

 surrounding olivine grains. The material of the plates is bright 

 green in color, and it is slightly pleochroic. Its highest observed 

 extinction is 36 , and its most common inclusions are grains of 

 magnetite and masses of limonite. Most of the magnetite is 



