MUSEUM OF COMPARATIVE ZOOLOGY. 35 



tact of one sedimentary rock with anotlicr. The ore at the junction is 

 magnetic. Tiie question whether this is an intrusive or sedimentary 

 rock has another side than the simple scientific one. It makes a great 

 difference in the mine whether this is a simple overlying metamorphosed 

 sandstone, as Mr. Brooks places it, or a later intrusion cutting the ore 

 below. This latter case opens up numerous questions that the practi- 

 cal man can only disregard to his cost, sooner or later, but which have 

 nothing to do with the present discussion. 



As this rock seems to belong to the granites, it will be described 

 under them (p. 54). Should future research show that all of the 

 " quartzite" of Republic is the same as the tongue is, it would have a 

 bearing on the proximity of the magnetite and martite to it. 



In like manner, passing to other mines where secondary intrusions are 

 more abundant, the magnetite becomes a more prominent feature. It 

 seems, so far as we have seen, that the magnetite and martite are 

 directly proportioned to the amount and proximity of eruptive rocks, 

 extravasated since the ore was in situ. 



East of the Old Washington mine, Humboldt, the actinolite schist 

 (310) and jasper with its ore (magnetite) were seen within one hun- 

 dred feet of one another, but dipping in opposite directions ; three hun- 

 dred feet farther west ihey both dip in the same direction, while a few 

 rods away the magnetite (311) with the jasper is seen breaking irregu- 

 larly thi'ough the schist (312), and sending tongues into it. 



The intrusive nature of the ore was well marked here, but it was very 

 difficult to procure any hand specimens showing it (313, 314, 315, 316). 

 An attempt had been made to mine the ore at this locality. Three rods 

 to the east the ore and jasper were seen intruding in long tongues and 

 sheets between the planes of the schist, as well as breaking across the 

 lamination (317, 318). The same structure is very well marked in the 

 rocks that form the bluffs between the Old Washington and Edwards 

 mines. The ore with quartz (325) sends long tongues up between the 

 lamince of the actinolite schist (326), Although in part these coincide 

 with the bedding, yet in many places they break obliqtiely across it, 

 showing their intrusive character in an unmistakable manner. 



It would seem that this intrusion between the planes (intrusive 

 sheets) in this locality probably took place when the schist was in a 

 somewhat unconsolidated condition, the intruded rock serving as an effi- 

 cient agent of metamorphism. The effect produced by the ore and its 

 relations to the schist are not such as we should expect, or are accus- 

 tomed to see, when a rock is intruded through one indurated as the schist 



