78 THE SIDEROLITES AND PALLASITES. 



The magnetite has diminished in amount, and for the most part forms a discontinuous 

 sponge. This discontinuity appears to have arisen from tire solution of the magnetite 

 along the borders of its fissures and edges, which solution m some places has removed 

 nearly the whole of this mineral. 



The removed portions are replaced by actinolitic material. The olivine is generally 

 surrounded by a border of actinolitic material, whose general relation is shown in Plate II. 

 figure 1, the black portion representing the magnetic sponge, the brownish parts the 

 smoky, fissured olivine, and the gray and white portions the secondary actinolite. 



In this the actinolite band is rejiresented by the uncolored portion surrounding the 

 olivine, and in its turn inclosed by the magnetite. Little granules remain in the actin- 

 olitic material, part of which are shown in the drawing. The olivine as before is 

 "smoky," marking apparently the first stage in its alteration. 



Nos. 1007 and 1008 are both in the section and hand specimen similar to the pre- 

 ceding number, only in some portions the alteration has not extended (juite so far. 



No. 1006 is somewhat more changed than No. 1002. The section is partially 

 crossed by greenish actinolitic material, which replaces nearly all of the original matter. 

 In other portions the original structure still remains, the olivine being partly replaced 

 by actinolite, etc. The centres of the olivine are in part only dark smoky-brown, but 

 in others are altered to a reddish-brown serpentinous like product. 



The section is stained by a greenish, yellowish, and brownish product in many places. 

 This section serves as the last Imk in the chain connecting the specimens which contain 

 unaltered olivine with those in which the olivine is entirely changed. 



No. 1001 is from the same locality as No. 1000, but nearer the centre of the hill. 

 This is a dark greenish black rock, showing a greenish serpentinous groundmass sprinkled 

 with titaniferous magnetite. Bounded and irregular patches of green serpentine, com- 

 paratively free from the magnetite, are irregularly distributed in the groundmass. In 

 weathering, the magnetite is left projecting in a cellular sponge-like mass. 



Section : This, like the preceduig, is composed of an irregular sponge-like mass of 

 magnetite, with the interspaces filled with pale greenish and grayish mineral matter. 

 While the structure of the section is essentially the same as that of Nos. 998, 999, and 

 1000, the olivine is entirely rejjlaced by serpentine. The forms of the olivine grains 

 remain the same, and the fissures by which they were traversed are marked by magnetite 

 grains arranged in lines extending through the serpentuie. With a low power m com- 

 mon light the section is almost undistinguishable from one containing unchanged olivine. 

 With a higher power, or in polarized light, the fibrous structure of the serpentine shows 

 itself. The polarization colors are dull, and the platy fibrous masses show some resem- 

 blance to talc, as well as being obscured when the fibres are parallel to a diagonal of the 

 crossed nicols. The original fissures of the olivine show well both in common and polar- 

 ized light. Magnetite is included in the serpentine, not only in original grains, as seen 

 in the ohvine of No. 998, but also in scattered dust-like granules and irregular masses, 

 or in lines along the fissures. The latter magnetite is apparently a secondary product 

 formed duruig the conversion of the olivine to serpentine. The main magnetite sponge- 

 like mass is not quite so closely united as in No. 998, neither does it occupy so great an 

 extent of the section ; i. e., the percentage of magnetite is somewhat smaller in this por- 

 tion of the hill, and continues less in all parts of the hill not immediately adjacent to 



