These constitute interesting objects for the microscope. The first mentioned is calc spar, having perfect rhombohedral cleavage; it is transparent, or translucent, apparently coloured topaz yellow or mnber, by the intimate diffusion of oohreous iron throughout. The distribution of the colouring iron oxide through the carbonate of lime is minutely microscopic, in tangled interlacing curved lines, many of which, if not all of them, are hollow tubes not unlike maccaroni or vermicelli, but, although this form of iron oxide is distributed through the calc spar in such close masses, it does not seem to affect the cleavage planes of the latter, which separate as readily as if no ochreous iron had been present.