112 



BULLETIN OF TEIK 



has attracted considera- 



phyrs were seen that were apparently in the form of dikes intruded 

 since the lava flows (624, 626, 627, 628). 



The form of lava locally known as " ash-bed ' 

 ble attention. It has been described in various ways, generally being 

 regarded as fused sand and trap. Latterly Prof. Pumpelly has described 

 this as "volcanic scorise buried in the littoral sand."* At the present 

 time the ash-bed can be best studied at the Copper Falls mine, whore it 

 is largely mined for the copper it contains. After a careful study of it, 

 we conclude that the ash-bed is a very scoriaceous, and, conjparatively 

 speaking, somewhat thin lava flow. It does not possess the ordinary 

 characters of the other flows of Keweenaw Point, but seems to be largely 

 made up of clinkers and scoriaceous masses. It appears to have flowed 

 in a more or less fragmental condition, forming a black, rough, loosely 

 aggregated, scoriaceous, cinder and lava sheet, similar to those described 

 by Prof. Whitney in his "Report on the Geology of California," f and 

 by Prof. Palmieri in his "Account of the Eruption of Vesuvius of 



1871-72." + 



At the time of the flow, or since, the interstices were filled with de- 

 trital mud. The various parts of the flow seem to be connected in the 

 main, and do not form to any great extent true pebbles. 



That the rounded fragmentary portions derive their structure from 

 the cooling of a fluid mass, and not by water action on the melaphyrs, 

 is shown by their cellular structure in the interior, and solid crust on 

 the exterior, the same as similar modern lavas have. Water-worn peb- 

 bles of melaphyr, as seen at some of the old burrows of the Hancock 

 mine in a detrital deposit (832, 833), which to the casiuil observer 

 resembles the ash-bed, have the cellular structure extending throughout 

 the mass, and are destitute of any crust. 



The saAie flow exists at the Petherick and Old Phoenix mines, while 

 a similar formation is mined at the Atlantic. Besides the true ash-beds, 



4 



there seem to te confounded with them certain sedimentary deposits 

 composed of rotmded water- worn pebbles of melaphyr held by littoral 

 ,sand. These forms pass- into a more siliceous conglomerate or sand- 

 stone, or not, according to the length of time that elapsed before the 



They are abundant west of Houghtf#i, and, so far as 

 I could find, are the only formations existing at the Hancock mine that 



* Proc. Am. Acad., XIH. 283. 



t 1865, Vol. I. p. 267. 



4 Annali del reale Oascrvatorio meteorologico Yesvaviano, (Napoli, 1873,) p. 28. 

 The same, translated by Robert Mallett, (London, 1873,) p. 103. 





succeeding flow. 



^ 





