130 The American NaturaliM. [February, 



vine basalts corresponding to geological masses intermediate in charac- 

 teristics between volcanic sheets and necks. In many of the neck 

 rocks the hornblende is seen to have been partially resorbed and 

 changed to augite. The continuation of the resorbtive process until 

 every trace of the hornblende was dissolved, may account for the ab- 

 sence of the mineral in the sheet rocks. 



Petrographical Notes.— In an article whose aim is to call forth 

 more accurate determinations of the feldspars in volcanic rocks, and 

 one which gives a practical method for making this determination, 

 Fouqu£ 8 has described briefly the volcanic rocks of the Upper Auver- 

 gne, the acid volcanics of the Isle of Milo and the most important rocks 

 in the Peleponeses and in Santorin. Among the varieties described 

 are doleritic basalts, andesitic basalts, labradorites, andesites, obsidians, 

 trachyte andesites, phonolites. andesitic diabases, rhyolites, dacites and 

 normal basalts. The labradorites are composed largely of microlites 

 of labradorite with a few augites and tiny crystals of olivine in an al- 

 tered glassy base. In all these cases the author has shown that the 

 rocks contain several different feldspars at the same time, and in eacli 

 case he has determined their nature. The method made use of in the 

 determination is based on the observation of extinction angles in plates 

 cut perpendicular to the bisectrices. 



In a well written article on complementary rocks and radial dykes 

 Pirsson" suggests the name of oxyphyre for the acid complementary 

 rock, corresponding to the term lamprophyre for the basic forms. He 

 also calls attention to the fact that the dykes radiating from eruptive 

 centers are usually filled with younger material than that which com- 

 poses the core at the center. The dykes cutting the central mass will 

 generally be oxyphyres and the more distant ones lamprophyres. 



Cordierite gneisses are reported by Katzer 10 from Deutshbrod and 

 Humpolitz in Bohemia, where they are intruded by granite veins, and 

 where masses of them are occasionally completely surrounded by gran- 

 itic material. 



In the examination of a large series of granites and gneisses from the 

 borders of the White Sea, Federow 11 discovered that garnet is present 

 in large quantities when plagioclase is absent and vice versa. 



In a general article on the Catoctin belt in Maryland and Virginia, 



" Bull. Soc. Fran.-, d. Min., XVII, p. 42i>. 



