CONTACT METAMORPHISM BY INTRDSIVES. 205 



by contact action they are suiTounded on all sides by tlie dolerites, being' 

 in fact inclusions, but without the immediate contacts exposed. Such 

 inclusions are rather numerous on the east side of the Michigamme River, 

 from sec. 29 N. to sec. 8, T. 43 N., R. 31 W., near the boundary line 

 between the Mansfield slates and the dolerites. 



The Mansfield slates are uniformly rather fine grained, and the contact 

 products are also fine-grained rocks, which still show in some cases the fine 

 banding- of the orig-inal slates. They are very dense "hornstone"-like 

 rocks, have a splintery and at times almost conchoidal fracture, and vary in 

 color from light to very dark gray and greenish The weathered surfaces 

 in almost all "cases are coA^ered by a thin white to light-yellowish crust. 

 This weathering brings out very clearly the banded and spotted character 

 of some of the rocks. 



The mineralogical components are quartz, feldspar, biotite, chlorite, 

 white mica, actinolite, rutile, epidote, spliene, and iron oxide. Quartz is in 

 very minute grains. Mucli of the feldspar shows fine striations, but owing 

 to the minute size of the grains their exact characters are not determinable 

 with the microscope, although from the very high percentage of soda shown 

 by analysis to be present in the rocks the conclusion is drawn that they 

 are grains of albite. 



Biotite is ^^resent in small quantity in some of the contact products. 

 This production of secondary biotite has been noted as rare for "diabase" 

 contacts.^ Plates of chlorite and white mica and needles of actinolite, the 

 latter of much larger size than the individuals of the other minerals men- 

 tioned, lie scattered through the fine-grained mass of feldspar and quartz. 

 Usually they are gathered together in bunches and sheaf-like or radial aggre- 

 gates, but they occui' at places in isolated individuals. Scattered through 

 the slates is unmistakable rutile in coarse crystals with pyramidal ends. In 

 only one case are the crystals very fine, and in that case they approach 

 closely the appearance of needles in the clay slates (Thonschiefernadeln). 

 These needles are commonly aggregated into tangled and roughly radial 

 growths. The needles show very pretty knee- and heart-shaped twins. 



Various combinations of the minerals occur, and the structures which 

 accompany the combinations likewise vary. As a result of these variations 

 there are found the different types of contact products described as spilosites, 



' Mlkroskopische Physiographie, by H. Rosenbusch, Stuttgart, 1896, Vol. ii, p. 244. 



