Gordon and Graton — Formations in JVevj Mexico. 393 



pods are plentiful just above the most eherty beds. Both the 

 corals and the brachiopods belong in the Richmond division of 

 the Upper Ordovician. 



The character of transition of the rocks of this system into 

 those above i§ not yet wholly established. Locally the upper 

 portion of the limestone series is coarsely brecciated, the beds 

 being composed of blocks of various sizes up to 10 feet in 

 breadth and the spaces between filled with calcareous material 

 derived from the disintegration of the same beds. The dis- 

 turbance appears in some instances to have involved a bed of 

 quartzite or gritstone, fragments of which occur along with 

 those of the limestone. 



In Sierra County the surface of the limestone is undulating 

 or billowy, and over considerable areas the topmost layer is 

 altered to a highly siliceous rock, in places quartzose and drusy, 

 in others constituting a flint breccia. In some cases the silicifi- 

 cation and brecciation was observed to extend along cracks 

 downward into the lower limestone beds, which fact, taken in 

 connection with other facts touching the nature and occurrence 

 of the phenomena, warrants the conclusion that the silicilication 

 has been effected by hot waters coming up from below and 

 spreading out along the contact with the overlying impervious 

 shales. The silver deposits at Kingston and in adjoining dis- 

 tricts occur in the upper part of this limestone. 



Whether the coarse brecciation and billowy surface repre- 

 sent a kind of unconformity caused by a retiring of the waters 

 following a period of limestone formation, as suggested by 

 Chamberlin,* or whether it is due to underground solution, 

 crustal movement, or to some other cause, we are as yet unable 

 to decide. 



At Silver City a perplexing feature of different character 

 arises. In the 870 feet of limestones which overlie the Cam- 

 brian quartzites, there is no visible stratigraphic break ; and in 

 the upper 420 feet, no lithologic difference has been noted, yet 

 in the lower part of this 420 feet are found Ordovician fossils, 

 while near the top a Silurian fauna is present. It should be 

 said that the rocks at this critical point are not so well exposed 

 as they are at most places 1 in the section, and that the 

 original character of the limestones is somewhat obscured by 

 impregnation of ore-bearing minerals and by their decomposition 

 products. It may be added that in the Clifton district Devo- 

 nian strata rest with apparent conformity on the Ordovician 

 limestones, while in the Bisbee region both Ordovician and 

 Silurian are absent although there is no structural break in the 

 stratification. 



* Geology of Wisconsin, vol. 1, pp. 138-140. 



