824 
The American Geologist. 
May, 1896 
Figure 1. 
In the figure numbers 1 and 2 represent thin sections of 
granite in which the ferro-magnesian constituents are seen to 
be more opaque than the feldspar or the quartz. Number 3 
is a thin section of hornblende gabbro in which the plagio- 
clase is seen to be less opaque than the hornblende. Several 
minerals placed in a card-board box are represented in the 
•centre of the figure. The circle shows the position of the rim 
of the box. Numbers 4, 5 and 6 are crystals of quartz, beryl 
and garnet, respectively, 7 three grains of corundum (ruby), 
MINERALS AND THE ROENTGEN RAYS. 
B> W. G. Miller, Kingston, Ontario. 
There are considerable differences in the opacities of min¬ 
erals towards the Roentgen rays. This is best brought out, 
as shown in the accompanying cut (figure 1), from a photo¬ 
graph taken by the writer during February, in thin sections 
of tocks having about the requisite thinness for the ordinary 
examination under the petrographical microscope. If much 
thicker pieces of rock are used the different mineral grains 
interfere with one another and the results are indefinite. 
