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 laminae does 

 not correspond with the cleavage, but is parallel to planes of the obtuser 

 rhombohedron — i R. Quite often two sets of these laminae 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 laminae might be at once visible. The appearance of 

 crystals exhibiting these laminae between crossed Nicol prisms is seen 

 in Fig. 5 on PI. 6. In some of the grains two systems of laminae 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 



