INTRODUCTION. 9 



homogeneous will be seen to be so filled with matters foreign to its substance as to 

 make a marked proportion of its mass. For example : at Hanover the schists are filled 

 with pretty, perfectly crystallized red garnets. A section shows that these garnets en- 

 close enough quartz, in little colorless grains, to make one third of their mass. Numer- 

 ous other cases will be described in this work. The minute crystals, that commonly 

 exist as impurities in other minerals, and which are so small that it is impossible to 

 determine the name of the species to which they belong, are called crystallites. The 

 nature of many of these crystals is suspected but not known. Zirkel divides them 

 into bellonites, which are colorless, and trichites, which are black, — terms that have 

 little use in the study of such minerals and rocks as ours. The microscopic crystals 

 which exist as impurities, but which possess either form, optical characters, or other 

 properties by which they can be determined, are called microlites. 



The cavities that minerals contain and the contents of them have been studied by 

 many investigators. The microscope shows that crystals, either isolated or imbedded 

 in rocks, are often filled with minute cavities, which contain fluids or crystals, or both. 

 As the presence, and the nature of the contents, of these cavities give important indi- 

 cations of the origin and former conditions of the crystals and the rocks in which they 

 are found, they are worthy of careful attention. Our rocks furnish some remarkable 

 examples of inclusions of this nature. Cavities containing water, and often salt crys- 

 tals, and others containing liquid carbonic acid, are to be described. I have seen no 

 rock which is so filled with cavities containing the latter fluid as one which has been 

 found in the progress of this study ; and sections showing this interesting inclusion 

 may hence become readily accessible to all. 



Crystalline Outlines. The first thing that will be noticed when unknown minerals, 

 as they occur in rocks, are microscopically examined with reference to their determina- 

 tion, is the outline of the sections of the crystals. Where crystals are cut at hap- 

 hazard, as they usually are when scattered through a rock, it must be borne in mind 

 that very variously formed sections can be cut from the same crystal. For example : a 

 square, a rhomb, a triangle, or a hexagon can be cut from a dodecahedron. Yet, as 

 the cr}stalline forms of imbedded minerals are commonly simple, and not subject to 

 very much variation, the examination of the various sections of the same mineral that 

 are commonly obtained in the same preparation, often enables one to form some judg- 

 ment in reference to the form of its crystallization. Crystals are, however, often much 

 distorted when found in the narrow confines of rocks ; and hence very odd and striking 

 outlines are met with, which are hard to refer to a crystalline form, and though plainly 

 crystalline, as shown by their other physical properties, minerals have often no defined 

 outline whatever, but exist as grains or irregular masses ; hence experience and judg- 

 ment are necessary in drawing conclusions from crystalline outlines. They are, how- 

 ever, very helpful ; and sometimes it is desirable to measure the angles of a crystal. 

 In the microscope referred to, the stage is revolvable ; the tube of the instrument is 

 capable of being exactly centered, so that the revolution of the stage does not move 

 the centre of the field of view ; and then, by means of the spider lines in the micro- 

 VOL. IV. 2 



