lOO 



Cincinnati Society of Natural History. 



The rock near the center of the dyke is less compact, and 

 its weathered surface is dotted with hornblende needles. 

 While not itself distinctly foliated, this rock is largely com- 

 posed of finely fibrous secondary hornblende, much pitted 

 with particles of quartz. In some cases, rocks from this 

 dyke show the hornblende in disseminated acicular needles 

 of great sharpness of outline, and often arranged so nearly 

 parallel as to produce a foliation. Such varieties resemble 

 the nadel-diorit of Guembel,* and not infrequently contain 

 both biotite and chlorite. * 



Frenchtown Diorite. — In the thin section of the dioritic 

 dyke (39) farther south, near Frenchtown, the foliation is less 

 pronounced than in the last series. The hornblende occurs 

 in decidedly uralitic crystalloids, of a pale green color, which 

 grows darker toward the edges. This mineral forms a large 

 proportion of the rock, and is undoubtedly secondary. The 

 intersticial substance is almost entirely feldspar, which is 

 changed to an aggregate of fine epidote grains. A little 

 quartz is present, but no magnetite or other accessor}' miner- 

 als were noted in the section. 



In a thin section of the Harrisville dyke the characters are 

 found to agree very closely with those of the Frenchtown 

 diorite. It, however, contains more quartz, and the horn- 

 blende is less fibrous. This last mineral shows similar darker 

 peripheral portions. Biotite is present, and secondary epidote 

 occurs in the feldspar in much larger masses than in the 

 Frenchtown diorite. 



The examination of the sections shows that all the speci- 

 mens collected are quart z-diorites. These may be further 

 divided into two groups : hornbleude-quartz-diorite, which 

 includes most of the thin sections, and biotite hornblende- 

 qmrtz-diorite. There is nothing in their mineralogical com- 

 position opposed to the hypothesis that they are dynamically 

 metamorphosed gabbros. 



-Ostbayerisches Grenzgebirge, p. 349, quoted by Rosenbusch, Massigen Gesteine, 

 p. 106, 1887. 



