10 BULLETIN .OF THE 
Augite Diorite.—This rock, though quite similar 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 ¢ = b>>q. The 
sections have in general distinct outlines parallel to the fundamental 
prism and the clino-pinacoid. A common feature of the hornblende 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,* Sjégren,> 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 
A. Streng. Neues Jahrbuch fiir Mineralogie, etc., 1877, p. 153. 
G. W. Hawes. Mineralogy and Lithology of New Hampshire, pp. 57, 206, 
R. D. Irving. Geology of Wisconsin, ITT. 170. 
C. R. Van Hise. Am. Journ. Sci. [3], XXVI. 29. 
H. Sjogren. Neues Jahrbuch fiir Mineralogie, etc., 1884, I. 82 (Ref.). 
A. v. Lasaulx. Verh. d. Naturh. Vereins d. pr. Rheinl. u. Westf., 1878, p. 171. 
G. H. Williams. Am. Journ. Sci. [3], XXVIII. 259. 
8 A. Schenck. Die Diabase des oberen Ruhrthals, ete. Diss., Bonn, 1884. 
® Quart. Journ. Geol. Soc., XL. 658, Plate XXIX. Fig. 3. 
10 Min. u. petr. Mitth., VII. 1, Plate I. Figs. 1-7, 1886. 
