No. 9. — Contributions from the Pctrographical Laboratory of the 

 Harvard University Museum. 



III. 



On Keratophyre from Marhlehead Neck, Massachusetts. 

 By John H. Sears. 



This interesting rock formation was first noticed by Prof. W. 0. 

 Crosby in the American Naturalist (Vol. XI. No. 10, 1877, p. 585), 

 where he says : " Near the middle of the southwest side of the harbor, 

 visible only at low tide, is a hard, whitish, fine-grained sandstone or 

 arenaceous slate. It overlies uncouformably the banded petrosilex 

 found on the shore." In the '* Occasional Papers of the Boston Society 

 of Natural History, III. Contributions to the Geology of Eastern Massa- 

 chusetts," Professor Crosby says again of Marblehead Neck (p. 263) : 

 " It is not generally known that this rocky peninsula, which may be re- 

 garded as lying on the extreme outskirts of the Boston Basin, includes 

 beds probably referable to the same horizon as the slate and conglomer- 

 ate on the south and west. Briefly stated the facts are as follows : 

 Near the middle of the northwest shore of the Neck, visible only at low 

 tide, is a hard, whitish, fine-grained sandstone or arenaceous slate ; it is 

 evidently largely feldspathic and turns yellowish on weathering. Por- 

 phyritically interspersed through the rock are clear, almost transparent, 

 rhomboidal crystals, from one eighth to one fourth of an inch long ; 

 these have been examined by Miss Hattie A. Walker and proved to be 

 orthoclase." 



The next notice of this rock i? in the Proceedings of the Boston 

 Society of Natural History (Vol. XXI. Part 3, p. 288), "On the Tra- 

 chyte of Marblehead Neck," by Dr. M. E. Wadsworth, in which he 

 says, " Near Boden's Point, on the northwest shore of Marblehead 

 Neck, there is to be seen, exposed between high and low tide, the re- 

 mains of a trachytic overflow." On page 290, Dr. Wadsworth says : 

 " One of the feldspars, porphyritically enclosed in the groundmass, was 

 obtained in the section. This is clear, glassy, and contains only a slight 



VOL. XVI. —NO. 9. 



