MUSEUM OF COMPARATIVE ZOOLOGY. 43 



the decomposition of titaniferous iron, as well as one or two that proba- 

 bly resulted from the decomposition of olivine or brown hornblende. 

 The quartz contains fluid inclusions. The groundmass is now composed 

 entirely of scales, plates, grains, and microlites belonging apparently to 

 chlorite, mica, and quartz, and with the exception of the feiTuginous 

 decomposition product no trace of the original structure and constitu- 

 ents remain. We regard the rock simply as a more highly metamor- 

 phosed condition of the " diorites " of the region ; but were it not for 

 its field relations we should not be able to tell its history from micro- 

 scopic examination. In such a case as this, with our present knowl- 

 edge, the microscope fails to give us any idea of its origin, whether 

 eruptive or sedimentary. Probably every lithologist, in examining this 

 section, would pronounce the rock to be sedimentary, yet we know it 

 to be eruptive, and probably in its original state a basalt. This well 

 illustrates the danger of deciding upon the origin of such highly altered 

 rocks by microscopic analysis alone, and calls attention to the necessity 

 of carefully ascertaining their relations in the field. 



One mile northwest of Deer Lake, west of the road, a high hill was seen 

 composed principally of "diorite" (232) ininning about north and south. 

 The contact of this " diorite " with the country rock is well marked on 

 both sides. On the west side it cuts a conglomerate, and on the east 

 a finer-grained fragmental rock (233). The contact is an irregular 

 eruptive one, and the country rock is indurated near it. At the south 

 pit of the Jackson mine, from which "soft hematite" is taken, a 

 diabase dike forty feet wide was observed running N. 30° W., and cut- 

 ting the ore. It was said that in mining its course had been found to 

 be variable. This diabase is a grayish brown, coarsely crystalline rock 

 composed of plagioclase, orthoclase, augite, brownish decomposed olivine, 

 magnetite, and ferrite (278). The plagioclase is but very little decom- 

 posed, and is beautifully striated. The diabase (280) near its walls was 

 very much decomposed, and its olivine, augite, and magnetite form a 

 dark brown decomposed mass cut through by the feldspar ci-ystals, which 

 are kaolinized, forming feebly polarizing masses. Were it not known 

 that it is part of the diabase, it could only be recognized as such from 

 the position and arrangement of its kaolin pseudomorphs relative to 

 the remaining portion of the section. 



In the Washington mine sevenil dikes of " diorite" and melaphyr cut 

 the ore, which is strongly magnetic in contact (327, 328) with them. 

 The melaphj-r (329, 34:2) encloses a horse of jasper and cuts the "dio- 

 rite." It (329) is composed of plagioclase, a little orthoclase, augite, 



