National Geographic : 1892 Mar 21
68 II. F. Reid-Studies of Muir Glacier. Gabbro of Mount Cook (Troctolite). In the collection of Alaskan rocks intrusted to me by Professor Reid and Mr Cushing there are no representatives of gabbro, but a single specimen of this type collected in the summer of 1890 by Mr I. C. Russell on the southern side of mount Cook has been sent me by Mr J. S. Diller, of the United States Geological Sur vey, and may appropriately be noticed in this place. This rock bears the closest macroscopic resemblance to the well-known forellenstein of Neurode in Silesia,* nor is the like ness less striking when the two rocks are compared under the microscope. The thin sections of the Alaskan rock which I have examined show an evenly granular aggregate of serpentine grains and a basic feldspar, which appears from its optical properties to belong to the labrador-anorthite series. The serpentine now contains no trace of the original olivine from which it has evi dently been derived. The feldspar is striped with broad twin ning lamelle, and shows evidence of considerable alteration, although none of the constituents of this rock exhibit any indi cations of having been subjected to any particular dynamic action. Around each serpentine grain is a border of compact greenish hornblende, which for considerable distances belongs to single individuals. To designate this peculiar modification of olivine-gabbro from which pyroxene is nearly or quite absent, the English petrog raphers employ a translation of the German term " forellenstein " (trout-stone) troctolite. As early as 1872 Professor Edward S. Dana proposed the name " ossipyte" to designate a rock from New Hampshire of the same mineral composition.t FINE-GRAINED DIKE OR SURFACE ROCKS. A goodly proportion of the specimens examined are fine grained porphyritic rocks, covering a considerable range of types. Their structure indicates that they belong tosm all masses, which in all probability break through the crystalline complex of more coarsely granular rocks above described in the form of dikes, or perhaps in some instances cover them as surface flows. These * Vom Rath: Pogg. Ann., vol. 95, 1855, p. 551; and A. Streng: Neues Jahrbuch fiir Min., 1864, p. 257. SAm. Journ. Sci., 3d ser., vol. iii, 1872, p. 49.
1892 May 15
1892 Feb 19