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THE JOURNAL OF GEOLOGY.

as defined by von Buch, and these had been given distinct names in accordance with the usual custom of distinguishing between the different varieties of a rock containing different characteristic mineralogical components. The years between 1860 and 1862, perhaps, marked the height of the wave of differentiation. After this time the classification of the numerous varieties took the direction along which it was to be carried farther by microscopical methods. Some of the hornblende gabbros, the forellenstein, many of the hypersthenites and some of the norites had been shown to be altered or fresh forms of true gabbros. The characteristics of the components of the two groups of the gabbros and the hypersthenites had been fairly well determined, and the similarity between many of the gabbros and the diabases had been pointed out.

The best résumé of the state of knowledge at this time concerning the rocks under discussion is to be found in Zirkel's[1] "Lehrbuch," published a year before the microscope was brought into use for the purpose of studying these rocks. Zirkel collected the observations of the different workers and incorporated them along with his own in such a way as to give an excellent impression of the value of macroscopic rock determinations, when undertaken by competent observers and aided by chemical analyses. He distinguishes as gabbros those rocks containing labradorite and diallage, at the same time agreeing with Bischof[2] in the view that the latter mineral is merely a variety of augite. Saussurite he regards as sufficiently characteristic of some gabbros to warrant their separation from others. He likewise looked upon smaragdite, which was thought to be an intergrowth of augite and green hornblende, as an essential constituent of some gabbros, and these he separated from the diallage gabbros under the name of smaragdite gabbros. The hypersthenites are described at some length, with the appended statement that many hypersthenites are probably gabbros. The

  1. ↑ F. Zirkel: Lehrbuch der Petrographie.Bonn, 1866, p. 112.
  2. ↑ Bischof: Lehrbuch der chemischen und physkalischen Geologie.Bonn, 1864. 2 Aufl. II, p. 654.
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