440 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 character- 

 istic mineralogical components. The years between r86o 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 micro- 

 scopical methods. Some of the hornblende gabbros, the forel- 

 lenstein, 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 gab- 

 bros 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 resume of the state of knowledge at this time con- 

 cerning the rocks under discussion is to be found in Zirkel's l 

 " 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 incor- 

 porated them along with his own in such a way as to give an 

 excellent impression of the value of macroscopic rock determi- 

 nations, when undertaken by competent observers and aided by 

 chemical analyses. He distinguishes as gabbros those rocks con- 

 taining 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 like- 

 wise looked upon smaragdite, which was thought to be an inter- 

 growth of augite and green hornblende, as an essential constitu- 

 ent of some gabbros, and these he separated from the diallage 

 gabbros under the name of smaragdite gabbros. The hypers- 

 thenites are described at some length, with the appended state- 

 ment that many hypersthenites are probably gabbros. The 



( 



X F. Zirkel: Lehrbuch der Petrographie. Bonn, 1866, p. 112. 



2 Bischof : Lehrbuch der chemischen und physkalischen Geologic Bonn, 1864. 

 2 Aufl. II, p. 654. 



