74 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



ERUPTIVE OR IGNEOUS. 

 The eruptive rocks of this reoion, besides the granites, which were 

 erujited during Archean time, are of Mesozoic or Secondary and of Tertiary- 

 age. The most important of these, both in magnitude of development and 

 in their relations to the ore deposits of the region, are the Secondary eruji- 

 tives ; the time of their eruption cannot, as explained in the preceding 

 chapter, be exactly fixed, but was probably toward the close of the Mesozoic. 

 The Tertiary eruptives, on the other hand, are of comparativel}' limited de- 

 velopment and have had no appreciable influence on the deposition of ore ; 

 their age is determined as such, not by any direct crossing of Tertiary beds, 

 of which no instances were found in the region, but from their lithological 

 character, their analogy to erujjtive rocks of known Tertiary age outside 

 of this area, and from the fact that they are later than the Secondary erup- 

 tives. 



SECONDARY ERUPTIVES. 



The earlier eruptive rocks occur mainly in the form of intrusive sheets, 

 often of great magnitude, which, having been forced up from below through 

 some more or less vertical vent or channel, have spread themselves out be- 

 tween the strata, generally following a definite horizon, but at times crossing 

 the stratification. The)' also occur in the form of dikes, this form being 

 most common in the underlying Archean rocks. There is no eWdence that 

 any of them were poured out upon the surface like the lavas of tlie present 

 day, but they must have cooled and consolidated under a great weight of 

 superincumbent strata, to which is doubtless in great measure due their 

 unusually crystalline character. 



They are with unimportant exceptions porphyritic in structure; that is, 

 the}' contain larger crystalline elements in a groundmass or matrix of finer 

 grain, as distinguished on the one hand from the granitic structui-e, in which 

 all the elements are crystalline and of comparatively uniform size, and from 

 Tertiary eruptives on the other, in which, while the structure mav be por- 

 phyritic, the larger er3'stals have a somewhat different development and 

 the groundmass is made up in great part of non-crystalline material. 



