MINERALOGY. 85 



we can conclude that these divisions in this mica are cleavages that were 

 produced in it by pressure resulting from some dislocating agency after 

 the mica was formed. The fibres that lie in these cleavages are only 

 much smaller cleavage strips. 



Beside enclosures of a different species of mica, other minerals are 

 found between or attached to plates of mica, and these minerals have all 

 the peculiarity of being flattened out into discs, ribbons, or net-works, — 

 forms induced on them by the crystallization of the mica. For example: 

 garnets are found in the mica, which, instead of being ordinary dodeca- 

 hedrons or trapezohedrons, are flattened discs; and when the mica is 

 scaled away till it is of the same thickness as the garnet, it forms a nat- 

 ural setting around the crystal, which, with its fine wine color, looks very 

 pretty in its yellow mica surroundings. Flattened crystals of tourmaline 

 are very common. The crystals are often so thin by reason of this flat- 

 tening, that they are quite transparent, and the effects of polarized light 

 can be observed by using two of them. The minute crystals that are 

 best seen with the aid of the microscope are the most perfect, and must 

 be extremely thin. Two little crystals, that by chance cross one another 

 at right angles, are shown in Fig. lo on PI. 2 as they were found in a piece 

 of mica from Grafton. Nature has here prepared for us the experiment 

 which is tried by every one in the beginning of the study of optics, — to 

 show the beautiful principle of polarized light. Other minerals are also 

 found. Magnetite, so thin as to be translucent; c|uartz, in little net-works 

 of flat crystals; feldspar, in thin, white plates; and beryls flattened into 

 ribbons are not uncommon. 



Muscovite is easily recognized in microscopic sections of the rocks by 

 its white color and its ready cleavage, which, in all but basal sections, is 

 very plainly evident in the straight lines which traverse it in but one 

 direction. Its sections are always black between crossed Nicols when 

 its cleavage is parallel to the plane of vibration of the light ; and all its 

 sections, whether basal or not, are four times dark in a complete revolu- 

 tion between the crossed Nicols. It has hence all the properties of an 

 orthorhombic crystal. It commonly is found in rocks in irregular bits 

 destitute of crystalline outline; but in the granites it does make efforts 

 to crystallize, as is often macroscopically evident. In the Roxbury gran- 

 ite, the microscope detects the most innumerable, very minute, yet most 



