668 THE JOURNAL OF GEOLOGY. 



to prove the igneous nature of the rocks were hardly of a char 

 acter to justify the conclusion, and that he was right in regard 

 to the so-called serpentine must be regarded as a mere matter of 

 chance. 



Description of the rock. — A hasty examination of many speci- 

 mens of the rock would seem to warrant Emmons' determination 

 of it as serpentine. It is dark green or black, massive, and very 

 fine grained or quite aphanitic, with rather waxy lustre, and 

 abundantly slickensided. Thus far it is precisely like serpen- 

 tine, and many specimens show no other prominent features. 

 But most of the rock is mottled with abundant white, vitreous 

 spots, of extremely variable size. Where these are large enough 

 to be clearly seen (and they may reach three or four inches in 

 diameter), it is evident that they are fragments of quartz. Speci- 

 mens in which the grains are small and evenly distributed closely 

 resemble a porphyritic rock with glassy or aphanitic groundmass. 

 In the presence of this quartz, there is a marked divergence 

 from the ordinary character of serpentine. 



The slickensides which are so abundant in hand specimens 

 are developed on a large scale in the rock exposed in the mine 

 pit. They run in every direction, and on this account most of 

 the exposed surfaces are slickensided, being, as a rule, curved 

 and showing a beautiful polish. Such surfaces measuring more 

 than one hundred square feet are not uncommon. A good 

 example appears in the centre of the pit upon a dome-like mass 

 of rock. 



Origin of the rock. — Accepting the evidence afforded by the 

 contact, that the rock must be igneous, a difficulty is met at once 

 in its apparently contradictory features. Its serpentinous 

 aspect suggests derivation from a basic rock, but conflicting 

 with such a supposition is the presence of much quartz. This 

 mineral is, moreover, so distributed that it cannot be accounted 

 for as inclusions, or, to any great extent, as secondary. These 

 facts, noted in the field, led the writer to a close examination of 

 the exposures in the hope of finding some portion of the rock 

 which, through less complete alteration, would give some clue to 



