10 BULLETIN OF THE 



Angite Diorite. — This rock, though quite sitnilar to the diabase, differs 

 from it in a loss of the ophitic structure, and in the appearance of brown 

 (basaltic) hornblende as the principal non-feldspathic constituent. In 

 some sections augite does not appear, owing to complete uralitization. 

 At other localities it comes into prominence, and there the rock may be 

 known either as a diorite or a diabase. 



The hornblende is for the most part the massive brown variety, which 

 is well characterized by its color, perfect cleavage, large optical angle, and 

 strong pleochroism. The absorption may be written c = b>>a. The 

 sections have in general distinct outlines parallel to the fundamental 

 prism and the clino-pinacoid. A common feature of the honiblende crys- 

 tals is the occurrence within them of cores of augite, which seem to show 

 either that the hornblende is derived from the augite by pseudomorphism, 

 or that the two minerals crystallized originally in their present relations. 

 Such pseudomorphism was first noticed by Streng ^ in 1877, and subse- 

 quently by Hawes,^ Irving," Van Hise,* Sjogren,^ and Von Lasaulx." 

 Remarkable instances of this change have been described by Professor 

 Williams,'' from the Cortlandt Series on the Hudson River, and by 

 Schenck,® from the diabase of the Upper Ruhrthal in Westphalia. The 

 former has shown the gradual passing of the augite into brown horn- 

 blende. The latter has described a further change of the brown to green 

 hornblende, while Von Lasaulx found in the diabase of Kiirenz that 

 the change of the augite was first to uralite, then to brown hornblende. 

 In the diorite which we are considering, the contact of augite and horn- 

 blende is a sharp line. No evidence of a gradation from one mineral to 

 the other was anywhere observable. The hornblende is in general very 

 fresh, while the augite alters readily to chlorite, so that in many cases only 

 a few scattered fragments of augite can be seen (Figure 2). It seems prob- 

 able, therefore, that these combinations are the result of parallel growth. 

 Teall^ has figured such growths in the Whin Sill, and Rohrbach" 



1 A. Streng. Neues Jahrbuch fiir Mineralogie, etc., 1877, p. 133. 



2 G. "W. Hawes. Mineralogy and Lithology of New Hampshire, pp. 57, 206, 

 Plate VII. Fig. 1. 



3 R. D. Irving. Geology of "Wisconsin, III. 170. 



* C. R. Van Hise. Am. Journ. Sci. [3], XXVI. 29. 



5 H. Sjogren. Neues Jahrbuch fiir Mineralogie, etc., 1884, 1. 82 (Ref.). 



•^ A. V. Lasaulx. Verb. d. Naturh. Vereins d. pr. Rheinl. u. Westf., 1878, p. 171. 



T G. H. Williams. Am. Journ. Sci. [.3], XXVIII. 259. 



8 A. Schenck. Die Diabase des oberen Ruhrthals, etc. Diss., Bonn, 1884. 



» Quart. Journ. Geol. Soc, XL 653, Plate XXIX. Fig. 3. 



10 Min. u. petr. Mitth., VII. 1, Plate I. Figs. 1-7, 1886. 



