I06 MINERALOGY AND LITHOLOGY. 



core. The crystals are ordinarily gray in their color. The White Moun- 

 tain Notch, Boar's Head (near Rye, in boulders), Charlestown, Troy, 

 Rochester, Farmington, Mt. Kearsarge, Mt. Pequawket, Mt. Monadnock, 

 and Andover, are a few localities where good crystals are found. 



The variety of andalusite called chiastolite is abundant in our state, 

 and many rocks are beautified by the very pretty figures with which 

 this mineral dots them. Sections across the ends of these crystals show 

 crosses, squares, etc., due to the regular arrangement of impurities, which 

 produce the black figures on a white ground, with which all are familiar. 

 The microscope shows that these black portions are made by little black 

 scales enclosed in the pure material, and, as a little heating destroys 

 them, they are supposed to be coaly, bituminous matters. Chiastolite 

 abounds on some parts of Mt. Washington, in Walpole, Albany, Alstead, 

 Langdon, and Rye, and poorer specimens are found in many other places. 

 Chiastolite is always found in argillitic rocks. 



If we obtain sections of crystals of andalusite we might suppose that 

 they would exhibit the characters of an orthorhombic crystal, but this 

 they rarely do. On revolving them between crossed Nicol prisms, no 

 point of maximum darkness is found, but the field remains light all 

 the time, and shows only the effects of aggregate polarization. This of 

 course indicates an alteration of the crystal ; and we have the material 

 for observing its progress and result. Even the hardest and apparently 

 most unaltered crystals under the microscope show alterations, as indi- 

 cated by the multitudes of minute crystals which radiate from the cleav- 

 ages. These cleavages are parallel to the faces of the primary prism, 

 which has an angle of almost 90°. These secondary crystals are made 

 visible by polarized light, and cause the section to assume a most beau- 

 tiful appearance when thus examined. Fig. i on PI. 6 represents a basal 

 section of such a crystal, in which this incipient alteration is shown. It 

 is of course impossible to indentify the mineral species into which the 

 andalusite is here turning; but last summer Prof. Brush brought some 

 andalusite crystals from New Preston, Conn., which he allowed me to 

 cut. The inner structure of these crystals is illustrated in Fig. 2 on PI. 

 6. Here it is evident that a complete alteration of the crystalline ar- 

 rangement has taken place, for the section figured is a basal one. None 

 of these crystals are dark between crossed Nicols when parallel with the 



