128 MINERALOGY AND LITHOLOGY. 
allel to the vertical axis of the crystal (extraordinary ray), are not 
so much refracted, and pass through with more ease than do those at 
right angles thereto. Hence sections of calcite exhibit absorption, 
and when viewed with the microscope with only the lower Nicol on the 
instrument, they are brighter and clearer in certain positions (that is, 
when the plane of the Nicol and the plane of the extraordinary ray 
coincide) than they are in others. With crossed Nicols, calcite gives no 
very brilliant colors, but when revolved on the stage of the instrument 
there are alternations of great brightness with the darkness. A peculiar 
silvery color is ordinarily obtained, which is very characteristic. Of 
course, basal sections are always dark between crossed Nicols; and in 
such sections, if the ocular is taken out of the instrument and the Nicol 
replaced, the black cross and colored rings can be seen. In quite thin 
sections, in order to see this, the higher powers must be employed. The 
perfect rhombohedral cleavage of calcite is always very evident in thin 
sections. 
Most especially in our marbles, and in the calcite that is found in the 
crystalline rocks as an apparently original product, an appearance is seen 
that resembles in a degree the polysynthetic twinning of feldspar. The 
calcite in such rocks possesses a laminated structure which is usually 
only brought into view when a thin section is brought between crossed 
Nicol prisms, and then it is very evident. The plane of the laminz does 
not correspond with the cleavage, but is parallel to planes of the obtuser 
rhombohedron —4 R. Quite often two sets of these laminz are seen 
crossing one another, and as there are three like rhombohedral planes, 
so it is evident that there might be at the same time a twinning parallel 
to all at once, and that if the crystal were cut in the proper direction all 
three sets of these laminz might be at once visible. The appearance of 
crystals exhibiting these laminze between crossed Nicol prisms is seen 
in Fig. 5 on Pl.6. In some of the grains two systems of laminz are 
seen at once. The different shades of the calcite depend, of course, 
upon the varying relations of the axes in the different grains to the 
plane of the light. This figure is drawn from calcite in the micaceous 
diorite at Stewartstown. It represents very well what is to be seen in 
sections of any of our limestones, and in the calcite enclosed in many of 
our rocks. Stelzner was the first to suggest that the entire irregularity 
