INTRODUCTION. During the last ten years, the number of ore deposits whiclx have been found at the contacts of intrusive igneous rocks with limestones has rapidly increased. This has been in part the result of the clearer understanding and the easier recognition of' thi.s type of depo, sits which the study of contact metamorphism and ore deposition has brought about. The investigation of theincreasing number of deposits has naturally led to a discussion as to the exact manner in which such

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... of ore have originated. A number of different hypotheses have been advocated for the formation both of 'the ores and the con, tact metamorphic minerals which have been developed by 6he intrusive igneous rocks in limestones. There has been n•ot a little controversy between those who advocate the different methods of genesis. As all geologists and mining engineers may not be entirely familiar with the arguments which have been presented by those who. favor one or the other of the proposed hypo.theses, and because more attention has been given to the origin of the associated ores. than to the study 'of the very extensive portions of the contact zones where little or no ore ha:s been found, t,he wrker feels that the following critical review of ,the existing theories is warranted. Inasmuch as the ores which are associated with these contact. zones occupy but a small portion of the contact area, more attention will be given to the non-metalliferous portion than to that which is composed chiefly of ore bodies. The sources of information are the various published reports which have been written .on those districts which are of economic importance, as x9