G. H. Williams — Peridoiites near Peekskill, JV. Y. 33 



Often however they form irregular patches scattered like little 

 clouds over the brown background.* 



The hornblende itself never shows any trace of crystalline 

 form. It fills the irregular spaces between the other constitu- 

 ents, a single individual often covering a space some inches 

 square. From its relations to the other minerals in the rock it 

 is evident that it was the last to solidify, while the great size 

 of the crystals would seem to indicate that the process of their 

 formation went on very slowly. If, as seems probable, the 

 portion of this rock now exposed cooled at a very considerable 

 depth below the surface, the minerals like olivine, hypersthene 

 and augite, which are commonly formed at comparatively high 

 temperatures, might have separated out of the magma, leaving 

 the remaining portion, which must have had almost exactly 

 the composition of the brown hornblende, in a more or less 

 pasty condition until the succession of a lower temperature, 

 more in accordance with the amphibole than with the pyroxene 

 arrangements of the molecules, finally allowed it to solidify in 

 its present form. The well-known fact that the same molecule 

 may crystallize at high temperatures as augite and at lower 

 ones as hornblende, renders this a possible explanation of the 

 curious structure of this rock. It is equally applicable to such 

 as contain no hornblende where bastite or diallage present the 

 same appearance. Here also it is the youngest mineral (i. e. 

 the one formed at the lowest temperature) which encloses the 

 others. In this case, however, complete solidification of the 

 rock may have taken place before the temperature was suffi- 

 ciently lowered to make hornblende a more stable form than 

 pyroxene. 



The hornblende seems particularly subject to alteration, 

 which is often far advanced before the olivine or the pyroxene 

 are materially affected. The first change which the hornblende 

 undergoes is a bleaching, accompanying which is the almost 



* The inclusions here described in hornblende, as well as those mentioned beyond 

 as occurring in the olivine, are identical with those which Professor J. W. Judd, 

 of London, has recently treated with considerable detail in his paper on the 

 Tertiary Peridotites of the "Western Islands of Scotland. (Quarterly Journal of 

 the Geological Society, vol. xli, p. 354, August, 1885.) This author considers all 

 of the minute, indeterminable bodies which are so common in the feldspar, hypers- 

 thene, diallage, hornblende and olivine of the more basic ancient eruptive rocks 

 as secondary in their origin. He thinks that at the great depths at which these 

 rocks were probably formed, the pressure imparted to the circulating waters such 

 an increased solvent action that cavities having the form of negative crystals 

 were produced in certain crystallographic planes, similar to the well-known 

 " aetzfiguren." Into these cavities he supposes certain ingredients, which had 

 been leached out of the mineral or out of other minerals surrounding it, to have 

 been deposited. To such a secondary process, which is almost always accom- 

 panied by the development of a glistening, bronzy luster on the planes in which 

 the negative crystal cavities have been formed, Professor Judd applies the name 

 " Schillerization." (]. c, p. 383.) 

 To the conclusion regarding the secondary formation of these well-known in- 

 Am. Jotxr. Sci. — Third Series, Vol. XXXI, No. 181.— Jan., 1886. 

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