ACADEMY OP NATURAL SCIENCES. 397 



portions of the miue. But the great mass of decomposed material which forms 

 Quail Hill as a whole, retaining as it does to so great an extent the original 

 structure of the country rock from which it was formed, can in no proper sense 

 be called a vein ; although its extent, when considered as a repository of the 

 precious metals, is something far transcending the size of ordinary veins. 



The gold and silver of these formations which have recently attracted so much 

 attention, and have become the object of extensive mining operations at Quail 

 Hill, seem to be distributed at the latter place, to a greater or less extent, 

 throughout the whole mass of the decomposed rock. The surface earth of the 

 bill, also, everywhere contains gold, which may be discovered by washing it in 

 the pan ; but this ceases to be the case on the hillsides as soon as the limits of 

 the decomposed rock are passed. Some of the gold, as stated by Prof. Silliman 

 in bis communication to the California Academy, already referred to, is quite 

 coarse ; but much of it is exceedingly fine and difficult to save in the mill. It 

 is a noticeable fact in the distribution of the precious metals at Quail Hill, that 

 the cupreous ores and the material in their viciuity have hitherto been found to 

 be always rich in gold and silver, and to contain chiefly, if not exclusively, the 

 coarsest gold. 



The distribution of the gold at Quail Hill is not uniform, the more slaty and fer- 

 ruginous portion of the decomposed rock being generally the richest in ore, while 

 the compact porphyritic kaoline contains but traces of gold, if any, and some 

 of the other and more compact rock is comparatively poor. The original distri- 

 bution of the sulphurets here seems also to have followed approximately the 

 same law — the kaoline containing in general but little trace of their existence, 

 while the more slaty rock is often full of their cavities. Hematite, as well as 

 the hydrated sesquioxide of iron, occurs here in small quantities ; and a curious 

 point in this connection is the fact that, while much of the best ore is very highly 

 charged with the hydrated sesquioxide, the hematite has been found hitherto to 

 contain little or no gold. The origin of the decomposed porphyry at Quail 

 Hill is a point of much interest, and it may be a question whether it is not the 

 remnant of an intrusive igneous dyke. The arguments in favor of this supposi- 

 tion consist in the entire dissimilarity in character and structure between it and 

 the surrounding material, as well as in the rarity of porphyry in the region 

 round about. In fact, I have nowhere else in this portion of the country seen 

 anything deserving of the name, while the whole texture and appearance of this 

 mass at Quail Hill are precisely such as would have resulted from the decompo- 

 sition in place of a true feklspathic porphyry. But however strongly these 

 facts may seem to argue in favor of an igneous origiu, it is not easy to reconcile 

 such a supposition with its mode of occurrence here. Other masses of similar 

 character may exist within the hill ; but so far as existing developments have 

 cut or uncovered the one of which I speak, the indications are that it is irregu- 

 lar in outline, quite limited in extent, and of approximate lenticular shape. 

 Moreover, in certain places, it seems to pass gradually into the eastern country 

 rock, without any distinct line of demarcation, the change in the texture of the 

 rock being even more gradual than the passage from the decomposed to unde- 

 composed material. At certain points, but a few feet from the eastern " wall," 



