W. S. Bayley — Spotted Rocks from Minnesota. 393 



neighborhood of eruptive masses it has been found that epi- 

 dote* is one of the most common of the new minerals pro- 

 duced. Moreover it is known that in these cases of limestone 

 metamorphism the eruptive rocks have added but little, if any, 

 of their substance to the intruded rock, except perhaps silica. 

 Their principal agency in the production of contact phenomena 

 has been heat. The percolation of silica-bearing solutions 

 through calcite-bearing sedimentaries was alone not sufficient 

 to produce the crystallized epidote described above. This is 

 evident from the existence of epidote-free calcite concretions 

 in rocks occurring at some distance from any eruptive mass, 

 while at the same time these rocks are typical indurated quart- 

 zites in which all the quartz grains have been enlarged by the 

 deposition of secondary silica around them (No. 11,461). 

 There was no necessity for the addition to these concretions of 

 any material from the eruptive rocks in order to change them 

 into epidote. They already contained the elements essential to 

 the formation of this mineral. These merely needed an oppor- 

 tunity to arrange themselves in the form of epidote — a very 

 stable compound under the conditions prevailing during con- 

 tact metamorphism, as so many investigations have shown. 

 This opportunity, it is believed, was afforded by the appear- 

 ance of the eruptive masses. The epidotic rocks and spotted 

 quartzites are always found in close proximity to masses of 

 eruptive origin. They moreover contain large amounts of 

 newly developed chlorite, a result which may likewise be 

 ascribed to contact action. It would seem, then, that we are 

 justified in regarding these epidotic rocks, particularly the 

 spotted varieties, as the result of the action of the intrusive 

 rocks, upon the sedimentary beds through which they forced 

 their way from beneath. 



Where calcite was scattered in little nests through the mass 

 of the fragmental rocks, epidote is now found in similar rela- 

 tions to the other constituents. Where the calcite was present 

 in spheroidal concretions enclosing quartz grains, feldspar, 

 chlorite, etc., there now occur little spheroidal bodies con- 

 sisting in large part of crystals of epidote. The hard envelopes 

 around these spheroids appear to owe their power to resist 

 weathering principally to the lack of chlorite in their composi- 

 tion. 



In closing I would acknowledge nry obligations to Dr. Geo. 

 H. Williams of the Johns Hopkins University for valuable 

 suggestions offered during the earlier portion of this study. 



January 19, 1888. 



* Cf. J. Roth : Allgemeine und chemische Geologie, Bd. i, p. 428 et seq. ; and 

 Rosenbusch : Mikroskopische Physiographie, 1885, Bd. i. p. 498. 



