410 THE CRYSTAL FALLS IROIT-BBAEING DISTRICT. 



Under the microscope the dolomites show no features of special inter- 

 est. They are thoroughly crystalHne rocks, chiefly composed of coarse 

 tyrains of dolomite with which is associated a considerable number of acces- 

 sory minerals. Of these the most important are tremolite, diopside, chlorite, 

 muscovite, phlogopite, quartz, and rutile, while apatite, tourmaline, pyrite, 

 and magnetite are rare. 



The dolomite is by far the most abundant constituent in most of the 

 slides, and furnishes the general background for the accessories. The shape 

 of the grains in many sections is decidedly oval, and the long axes lie in 

 the same direction, tluxs producing a foliation. 



Tremolite is abundant in some of the sections, and is entirely absent 

 from none. It occurs in long-bladed individuals and aggregates, usually 

 bounded by the prism, but one or both pinacoids are also sometimes present. 

 It includes portions of the carbonate background. Diopside is rather rare; 

 it occurs usually in small single individuals, with sharp crystal outlines. It 

 is sometimes surrounded by tremolite, from which it is distinguished by its 

 high obliquity of extinction and its almost rectangular cleavage. Partings 

 parallel to both pinacoids, as well as a transverse parting in prismatic sec- 

 tions, are also observable. Quartz occurs in irregular grains completely 

 interlocking with the dolomite, and in some cases with tremolite. In the 

 slides examined it is in all cases a secondary as well as a rare constituent. 

 In no case is there any indication that it is clastic. Chlorite is an abundant 

 constituent of some of the slides, while from others it is entirely absent. 

 Muscovite in little frayed plates is plentiful in some sections. Quite pos- 

 sibly some of these may be original clastic pai-ticles. The most interesting 

 mica, however, is phlogopite, which is very abundant in one locality near 

 the base of the formation. It occurs in large, cleanly bounded jjlates, 

 each of which is a multiple twin, and evidently a product of secondary 

 crystallization. Some of these plates have been strongly bent, thus showing 

 that the dolomite, like the quartzite, has been deformed since it crystallized. 



The thin sections therefore show that the rocks of this formation have 

 experienced even more nearly complete reconstruction than is shown in 

 the case of the quartzites, for here none of the constituents, except possibly 

 some of the smaller micas, are present in their original form. Also the evi- 

 dence for distm'bance after crystallization is of similar chai*acter and equally 



