SECONDARY EKUPTIVES. 75 



These distinctions are those that were in force before the introduction 

 of the use of the microscope in lithological study. The more intimate 

 knowledge of rock structure obtained by the microscopical study of rocks 

 has brought about many changes in preconceived ideas, which are increas- 

 ing every year, so that it seems merely a question of time as to when a new 

 system of classification may be required. Already the distinctions noted 

 above are true only of the most typical varieties of each, while between 

 these are transition members which often must be placed in the one cate- 

 gory or the other by some other distinguishing characteristic, such as time 

 of eruption, internal structure, etc. In the present work it has been judged 

 best to preserve the prevailing usage of designating the Secondary porphy- 

 ritic rocks in which the prevailing feldspar is orthoclastic as porphyries, and 

 those in which plagioclastic feldspars decidedly predominate as porphyrites. 

 When the porphyrite is entirely granitic or evenly granular it becomes a 

 diorite. 



On the general map of the Mosquito Range only two colors are given 

 to the porphyries, founded on two general divisions which have a geograph- 

 ical as well as a structural value. In the first of these is included the White 

 Porphyry and its closely allied form, the Mount Zion Porphyry, which are 

 the older and more nearly granular rocks, and which occur, with unimpor- 

 tant exceptions, only south of the north line of the Leadville map ; the sec- 

 ond includes all other varieties of the Secondary porphyritic rocks of the 

 region, which are generally younger and less uniformly crystalline, and 

 which do not occur south of the south line of the Leadville map. 



On the detailed map of Leadville and vicinity the principal varieties 

 of porphyry are each designated by a special color, the division " Other 

 porphyries " including those which could not, with absolute accuracy, be 

 brought into either of the other divisions. 



1 In the time that has elapsed since field work was completed and the maps colored, opportunity 

 has been had for studying more comprehensively the various Secondary eruptives in the course of 

 work carried on in neighboring districts, and it has been found that some of the varieties designated 

 oil the following pages as porphyry, viz, the Sacramento, Silverlieels, and Green porphyries, should 

 probably be classed as porphyrites. The reasons for this, as well as the detailed description of all the 

 rocks from a microscopical point of view, deduced from their study under the microscope by Mr. Cross, 

 will be found in Appendix A. 



