PETEOGEAPHICAL OHARAGTERS OP RANDVILLE DOLOMITE. 437 



The clastic pebbles give us striking proof of the general and severe 

 internal strains suffered by the dolomite, the effects of which have healed 

 over without a scar in the carbonate matrix. The pebbles are always 

 optically strained. Very often they are fractured and the parts separated, 

 and sometimes they have been reduced to small fragments. In these cases 

 the breaks have been completely healed by the flow or redeposition of the 

 groundmass in the interstices. These effects are found in greater or less 

 degree in every thin section. 



The oolitic varieties are very interesting under the microscope. They 

 consist of little oval or round areas, averaging 2 mm. in diameter, packed 

 together as closely as possible. Each oval consists of a single or compound 

 nucleus, surrounded by several thin and very even concentric layers. The 

 nucleus in a few cases is a single roundish quartz individual, evidently a 

 clastic grain. In most cases, however, it is composed of a great number of 

 minute quartz grains, or of several coarse calcite grains, with films of iron 

 oxide between. The arrangement of these separate quartz and calcite indi- 

 viduals is such as to indicate that they have filled interior cavities. The 

 surrounding thin layers are calcite in all cases. Sometimes two adjoining 

 nuclei, each Avithin its own rim of several layers, are together included 

 within a common series of shells. In one such case the outside rim trav- 

 ersed the edges of the rings surrounding one of the nuclei with a decided 

 unconformity, as if the latter had been eroded before the deposition of the 

 former. The oolitic structure, I believe, has not hitherto been noted in 

 limestones of undoubted pre-Cambrian age. 



SECTION IV. THE MAN^SFIELD FORMATION. 



The typical locality of the Mansfield formation is the Michigamine 

 River valley in the vicinity of the Mansfield mine, which lies a mile west 

 of the border of my field of work, and is described by Mr. Clements. The 

 same formation, however, is present in the Michigamme Mountain area, 

 where its relations to the adjacent formations are clearly defined. In the 

 Fence River area rocks of very different character and derivation occur at 

 the Mansfield horizon. These occur in typical development to the west, on 

 the Hemlock River, and are hence called the Hemlock formation. 



