466 A. A. JULIEX — AMPHIBOLE SCHISTS OF MAXHATTAX ISLAND 



Graphic analysis of the hornblende schist. — For the investigation of our 

 hornblende schist, the fifth method appeared most suitable. By meas- 

 urement of 50 crystals of hornblende, taken in succession as they oc- 

 curred in passing over several fields under the microscope in the ordinar}^ 

 thin-section, made on schist plane, the following average dimensions 

 were determined: Length (Q, 0.95 millimeter; breadth (6), 0.32 mil- 

 limeter; besides thickness (t), 0.09 millimeter, measured on a cross- 

 section. Then the side of cube of equivalent volume, 



d' = W lXbXt = 0.3013 millimeter. 



The area of face of such cube, equivalent to area of plate required in 

 proper cross-section, 



d" = 0.090 



To determine the section plane by division C of the fifth method, the 

 proper breadth of such plate, 



,, d" 0.09078 nno-'o 

 =—- = -TTTT^ = 0.09ooo9, 

 / 0.9o 



that is, about one-tenth of the breadth of the thin blades actuall}^ shown 

 on the schist plane. 



= nat sin 70° 20', 



the proper angle at which a thin-section should be sliced, from the schist 

 plane, to present the hornblende in exact areal proportion to its true 

 volume in the schist. 



In place, however, of slicing this additional thin-section, the division 

 A of the fifth method was resorted to, as attended with least error in a 

 rock of such homogeneit3\ In the usual thin-section on the schist plane, 

 instead of drawing the outlines of minerals, after the Delesse and Sollas 

 method, photomicrographs were made of each field studied, with low mag- 

 nifying power (figure 1, plate G2), and two prints, 10 b}' 12.5 centimeters, 

 obtained from each negative. On one, as a guide, the grains of each 

 mineral were marked as identified under the microscope, and the other 

 ])rint carefully cut up under a reading lens, and the separate parts, rep- 

 resenting each mineral, weighed together. Conditions of accuracy were 

 found in the considerable weight of even such a paper sheet, between 

 2 and 3 grams, in the slight difference in weight between any two sheets, 

 about 20 milligrams, and in the uniformity gained b}^ use of a sufficient 

 number. 



