Frazer.] ttX— [April 16. 



US to overcome, these plates are among the most faithful representations 

 of the facts as seen through the microscope which have yet appeared. 



With suitable apparatus and after some prefatory trials, I have hopes of 

 producfag more perfect results, and of obtaining sharp level photographic 

 outlines, which can be colored if necessary to correspond to five or six 

 positions of the analyzer during its rotation. 



[Note — In connection with this paper a series of thin slices of Con- 

 necticut Traps, made by Mr. E. S. Dana, of Yale College, the Penn 

 sylvania specimens referred to in the text, as also, photographs of maps 

 of York County and Gettysburg, and the positive picture on glass of the 

 slices of 136 diameter enlargement, were projected on the screen.] 



EXPLANATION OF THE PLATES. 

 Plate I. 



Fig. 1. This photograph was among the first made with- an y\ micro- 

 scopic objective. A portion of the edge of the section was included in the 

 field in order that the portion represented might be more easily recog- 

 nized and studied under the table-microscope. 



The enlargement is very nearly 34 diameters. The original is a 

 dolerite (No. 3) containing pyroxene (a), magnetite (b), plagioclase (labra- 

 dorite) (c), and some scattered needles of apatite (d). 



The previous description of the dolerite No. 3 from Beeler's farm 

 applies to this specimen. 



Fig. 2. The negative of this print was made in polarized light and is 

 another portion of Fig. 1, PI. IV. 



The object is a specimen of dolerite from Beeler's farm marked No. 4. 



The rock is seen to be a confused mass of crystal fragments consisting 

 of labradorite (a), pyroxene (b), and magnetite (c). 



Plate II. , 



Fig. 1. The negative of this print was taken with a ^ microscopic 

 objective, and the enlargement is about 136 diameters. The minerals 

 ■constituting this rock, (which occurs on Cemetery Hill, Gettysburg, 

 Adams County, Pa., and has been provisionally called Syenite,) are more 

 or less weathered, as their rough appearance, caused by their numerous 

 cavities, sufficiently shows. 



a. Crystals of feldspar. 



b. Hornblende. 

 . c. Magnetite. 



Fig. 2. This object is specimen 1 of dolerite from Beeler's farm, 3 

 miles S. W. of York, and is magnified 136 diameters. 

 a. Labradorite. 

 6. Pyroxene. 



c. Magnetite. 



