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norites of Scheerer are classed among the gabbros and the hypersthenites, and those of Esmark are said to belong partly with these and partly with the diorites.

In the year succeeding the appearance of Zirkel's book, as has been stated, Rose^[1] made the first microscopical examination of gabbros that has been recorded. He found among the Silesian gabbros two varieties, one of which is black and contains olivine, and the other green and free from olivine. Tschermak^[2] followed Rose with a description of some Austrian gabbros, and an announcement that many serpentines are altered gabbros, and that Streng's forellenstein is only an olivine gabbro. He concluded, further, that augite and diallage differ only in physical properties, and therefore that gabbro "ist eine Abtheilung des Diabas" (p. 168).

In the few years succeding Tschermak's paper several contributions of great importance were added to the literature of the gabbros. Zirkel^[3] recognized olivine varieties of these rocks among the Tertiary formations on the islands off the west coast of Scotland, and succeeded in showing that the hypersthenites described by Macculloch from the island of Skye contain no hypersthene. He further pointed out as important the fact that the plagioclase associated with diallage is rich in inclusions, while that associated with ordinary augite is free from them. In the same year Hagge^[4] continued the work that had been so ably begun by DesCloizeaux in 1864. He made a careful microscopic examination of all the important gabbro and hyphersthenite occurrences recorded, and reached a result very similar to that of Des Cloizeaux. He found that very many of the rocks