NATURAL SCIENCES OF PHILADELPHIA. T3 



April 7. 

 Dr. Jos. Leidy in the chair. 

 Sixteen members present. 



The Blue Gravel of California. By E. Goldsmith Under 

 the name of " Blue Gravel" the California gold miners, and espe- 

 ciall}^ the placer miners, understand a rock which underlies the 

 goldbearing alluvium of that State and part of Nevada. Speci- 

 mens of this rock were shown to me by John C. Trautwine, C. E. 

 It is stated that, whenever the goldbearing sand in many localities 

 in the two above-named States has been removed by the well- 

 known washing process, the "blue gravel" appears. It also con- 

 tains gold, which cannot, however, be extracted by washing, the 

 stream of water being unable to disintegrate the rock, which is a 

 compact composite one, and not, as the name "gravel" would 

 imply, a loose material. 



This so-called "blue gravel" is composed of two ingredients 

 widely differing in age, namely, of pebbles cemented together by 

 a lava. The pebbles are of all sizes. In the specimens alluded 

 to, they are ver3' smooth and rounded, and present no sharp edges 

 or grooves. Their color is externally brownish, with a slight 

 yellow or olive green, but without the least trace of blue. When 

 a pebble is scraped or cut with a knife, the fresh exposed surface 

 is bluish-gra3% The hardness is 4. From the general appearance 

 I infer that some of these pebbles were derived from the sedi- 

 nffentary rock, slate, and others from Hornblende rock. 



Entirely different in general aspect from the rounded pebbles is 

 the other part of the rock, which I have already stated to be a 

 lava. This appears to envelop the pebbles completely. When 

 we detach a pebble from the enveloping mass, a perfect impression 

 of the same is observed, having a beautiful lustre as if it had 

 been freshl3^ oiled. This oily lustre, indeed, characterizes the en- 

 tire enveloping mass. Some of the dull pebbles are as it were 

 externally polished by a coating of the volcanic glass which has 

 run over them. The lustre of the freshly exposed surfaces is 

 brilliant, but it sometimes loses its brilliancy and becomes dull 

 with handling. The color is dirty olive-green. This lava is very 

 brittle, so much so that the preparation of a thin plate for micro- 

 scopical observation is impossible. The hardness is equal to 

 apatite. 



The most distinguishing crystallization within the lava mass 



is a black mica, which is probably biotite. I noticed also a few 



grains of quartz, as well as flattened grains of bright yellow gold. 



These are all observable by the naked eye, and, with the pocket 



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