PETROGRAPHY, 



BY WHITMAN CROSS. 



INTRODUCTION. 



The eruptive rocks of the district embraced by this report are naturally divisible 

 into two groups, according to age. Although the age of neither group can be exactly 

 defined in geological time, the larger and more important one is unquestionably 

 older than the period of disturbance which produced the great faults and folds described 

 in other parts of this volume, while the other group is younger. In this district, rocks 

 of the former group penetrate the Tipper Coal Measure strata; in adjoining regions 

 they occur in similar manner in the Trias; and masses of nearly identical character 

 are found in the Cretaceous of districts not far removed from the Mosquito Eange. 

 The conclusion that the rocks of the older group are of late Mesozoic age seems war- 

 ranted by all that is known concerning their occurrence. In regard to the period of 

 dynamic disturbance, it has already been stated in Chapter II that the known evidence 

 places it at the beginning of Tertiary time. 



It is plain, then, that the rock groups mentioned might be considered the direct 

 equivalents of the Tertiary and Pre-Tertiary divisions of many writers, but it is thought 

 best to refer to them simply as the older and the younger groups, and by this division 

 it is intended to express merely the actual relationship as to age which is shown 

 by the observed occurrences. The question concerning the possible influence of age 

 upon the structure of these groups cannot be fully discussed at the present time, 

 because the rocks of other districts in Colorado, the study of which has been under- 

 taken, form with those of the Mosquito Range a connected series, requiring a correla- 

 tion of observations upon all of them before justifiable conclusions can be drawn. 



All of the older eruptives and some of the younger series are fully crystalline, 

 although few of them are typical granular rocks, and the structural forms presented 

 are such as render advisable some statement as to the sense in which the terms " gran- 

 ular" and " porphyritic" are used in the descriptions that follow. When these terms are 

 applied with the old and natural meaning, to designate certain universally recognized 

 rock structures, it is probable that the groups formed by the application will be prac- 

 tically the same, whether the attempt is made to accurately define the boundary line or 



319 



