74 PROCEEDINGS OF THE ACADEMY OF 



lens, I could detect nothing more. When small fragments of 

 irregular form were placed under a more powerful magnifier, the 

 black mica appeared smoky by a transmitted light, or green when 

 in very thin platp.s, and most of the material seemed to be a green 

 amorphous glass, having no effect on polarized light. The specific 

 gravity I found to be 2.48. The silica I determined by an analy- 

 sis to be 65.48 per cent. We might presume that the amount of 

 silica should be greater than that obtained, but we must keep in 

 mind that the biotite present contains only about 40 per cent, of 

 it, and thus reduces the proportion ; however, the quantity indi- 

 cates that we must class this lava among the acidic ones. The 

 conclusion at which I arrive is that the so-called " blue gravel" 

 of California is a conglomerate of pebbles of various kinds ce- 

 mented together by an acidic lava in which crystals of mica (bio- 

 tite) and grains of gold are imbedded. 



How the gold came into the lava is a question of some difficulty. 

 Whether it was mingled with the pebbles before the lava ran over 

 the bed, or whether the gold was ejected from the volcano, I am 

 not able to decide. It would require observations on a variety of 

 specimens to arrive at some plausible theory. The specimens I 

 have seen had the gold suspended in the lava. The metal did not 

 touch the pebbles ; therefore, if the gold was present in the pebble 

 bed prior to the ejection of the lava, this latter must have raised 

 the metal from its bed, that is to say, a metal of s. g. 19.3 must 

 have been raised by a semifluid mass having s. g. = 2.48 ! 



Of course my observations are limited on these questions, but 

 since copper was ejected by the ancient volcano on Lake Superior, 

 may not gold have been similarly ejected in the case before us ? 



April 14. 

 The President, Dr. Ruschenberger, in the chair. 

 Twenty-two members present. 



Prof. Leidy called attention to the " Bulletin of the United 

 States Geological and Geographical Survey of the Territories, 

 Ko. 2," presented this evening. It contains a "Review of the 

 Vertebrata of the Cretaceous Period found west of the Mississippi 

 River," by Prof. Cope. In this article he was quoted in such a 

 way as not fairly to express his original meaning. Thus, on page 

 7 of the Bulletin, reference is made to the Proceedings of this 

 Academy, 1856, p. 312, in which it is intimated that Thespesius 

 occidentalis was referred to the Mammalia, and regarded, per- 

 haps, as a Dinosaurian. In the Proceedings I have rather ex- 

 pressed the reverse, as I state of T. occidentalis, " among the 

 collection of vertebrate remains, are two apparent caudal vertebrae 

 and a first phalanx of some huge animal, which I suspect to be a 



