GEOLOGICAL EXPLORATIONS AND LITEKATURE— 1880. 73 



Observations of the same character were made at the New York, Jack- 

 sou, and other mines, and iu all of these the contacts of the jasper and ore 

 witli the surrounding schists were found to have the peculiarities of eruptive 

 contacts. The "bosses" of schist so frequently met with in mining- opera- 

 tions are likewise regarded as jjroofs of the eruptive origin of tlie ore. 



This structure evidently is consonaut with the theory of the eruptive origin of 

 the jasper and ore. They break obliquely up through the schi.st, and send otf 

 brauche.s, which, pursuing the same general course, leave wedge shaped masses 

 between them and the trunk. (P. 31.) 



At the Home mine, on the Cascade range, at the Pittsburg and Lake 

 Superior mine, on the same range, at the Lake Superior mine, Ishpeming, 

 and at a nuipber of other ]daces dike-like masses of the jasper are reported 

 as cutting schists and ferruginous sandstones overlying the ores. At the 

 Pittsburg and Lake Superior mine, "while in general these little dikes 

 follow approximately the bedding [of the quaiizite], they are seen not to 

 exactly do this, but cut the lamina^ oljliquely through much of their course." 



In order to determine, if possible, what was the original state of the 

 ore and jasper, the author examined thin sections of both. Of the jasper 

 he writes (p. 33): 



Microscopically this section is composed of a fine grapular aggregate of quartz 

 and hematite, and a more coarsely crystallized portion made up of octahedrons of 

 magnetite or martite, and of quartz of secondary origin. The quartz iu the first part 

 is largely filled with minute globules and grains of ore, which also occur in irregular 

 masses and in octahedrons. The quartz associated with the more coarsely crystallized 

 portion is water clear, and shows the usual fibrous granular polarization of secondary 

 quartz. Wherever the iron is in a distinguishable crystalline form it is iu octahedrons. 



Of other sections of jas[)er and ore he writes (}). 33): 

 The structure of the quartzo.se portion is like the devitrification structure of the 

 rhyolites and felsites. * * * The jaspery portion is finely banded, and shows an 

 apparent fluidal structure. We are inclined to regard the structure as fluidal, but iu 

 a rock so deeply colored it is difticult to make satisfactory examinations. 



It was impossible to determine whether or not the ores Avere all origi- 

 nally magnetic. In some cases the magnetites are believed to be secondary, 

 since the hematite where in contact with diorite dikes is often changed to 

 this mineral. 



