260 PROCEEDINGS OF THE AMERICAN ACADEMY 



140), wrongly designated these coarser rocks as diorites, I refer the 

 reader to these very faithful descriptions, by Mr. Marvine, of the ex- 

 ternal characteristics of these rocks, adding only the caution that, 

 wherever he mentions hornblende, it should read augite. 



As it is the object of this paper to describe the internal changes that 

 have taken place in these rocks, rather than a description of the rocks 

 themselves, the sequence in which they are described is that which is 

 best adapted to the immediate purpose. 



Earliest cnAKGES in Melaphtre. 



" The Greenstone." The rock, commonly knowa in the copper dis- 

 trict as " the greenstone," is the best type of this subdivision. Form- 

 ing a bed 400-500 feet thick, dipping northerly, its outcrop consists 

 of a continuous series of ridges and nearly vertical escarj^ments, often 

 several hundred feet high. These extend from the Allouez mine to 

 Point Keweenaw, and form both the most salient topographical feature 

 of the peninsula, and a well-defined horizon, to which the geologist and 

 the miner refer their measurements. The rock has suffered only to 

 the extent of a partial alteration of one of its constituents, — the 

 olivine. 



It is dark-green, greenish-black, finely crystalline, very compact, hard, 

 and brittle, and breaks with an uneven to semi-conchoidal fracture. 

 Porphyritic crystals, apparently of orthoclase, from \ inch in length 

 down, occur here and there, — one or two on the surface of a specimen. 

 Tliey are generally single individuals ; but sometimes twinned after the 

 Carlsbad law, as is shown by the oppositely inclined basal cleavage. 

 The powder of the rock yields to the magnet a beard of magnetite. 

 The specific gravity is 2.92-2.95. It is an important characteristic of 

 this rock, that its fi-eshly fractured surface is mainly occupied by spots 

 ^ to f inch in diameter, each of which reflects the light with a satin- 

 like sheen. The reflection is not carried to the eye from all the spots 

 at once : it is generally necessary to change the position of the speci- 

 men many times to observe the different reflections. Aiside from this 

 sheen, there is nothing, either in difference of color or texture, visible 

 to the naked eye, to betray the presence of these spots, which might 

 be called lustre-mottlings. 



To the naked eye, this phenomenon suggests, at once, interrupted 

 cleavage of large individuals of one of the constituents, as the cause ; 

 but, under a strong hand-glass, these reflecting surfaces show the same 

 granular texture and character as the rest of the rock ; and it is only 

 when examined under the microscope, with an objective of low power 



