MUSEUM OF COMPARATIVE ZOvLOV.V. 187 



It is rocks of this chanicter, as has been suggested by others, that give 

 us the most probable clew tt) the interior composition and structure of 

 the earth. 



The rock in the field shows, to our mind, no signs of structural planes 

 that should be referred to sedimentation. On one side the rock is mas- 

 sive and jointed, and on the other it is jointed in fine parallel planes. 

 This portion of the rock is more highly metamorphosed than the other, 

 and, as is usual in highl}^ altered eruptive rocks, joints parallel to cer- 

 tain lines of pressure occur. The writer has seen this structure in many 

 rocks that were indisputably eruptive, forming well-marked dikes in 

 other rocks. 



A rod away from the main mass of the iron ore, near one end, some 

 serpentine appears that cannot be directly connected with the other peri- 

 dotite. Microscopically its characters and structure are the same as 

 the main rock, and there is no reason to regard it as distinct. The rock 

 nearest to the peridotite is a mica schist some hundred feet away. It 

 shows no characters that would indicate the transition of the ore into it. 



The locality was visited by the writer in October last, in company with 

 Professor A. S. Packard, Jr., of Brown University, and Mr. T. S. Battey, of 

 the Friends' School, Providence, R. I. To the latter gentleman I am 

 especially indebted for a copy of the paper of the Rhode Island Society 

 before mentioned, and for other favors. 



This examination may serve as an illustration of the aid that micro- 

 scopical lithology may be to the practical side of life, since now, for the 

 first time since this rock has been worked, can the ironmaster who 

 wishes to use it approach understaudiugly the metallurgical problems it 

 presents ; whether he desires to employ the rock as a whole, or to con- 

 centrate the magnetite first. 



Cambridge, Mass., May 13, 1881. 



