ERUPTIVE ROCKS. 303 



things more difficult to draw a sharp and definite hne of separation than 

 in the case of the two characteristics ah'eady discussed. It is also to be 

 remarked that, whereas these structural distinctions have hitherto been 

 considered to be essentially a function of the age of the rocks, the studies 

 conducted during the present investigation tend rather to the conclusion that 

 these distinctions are primarily dependent on the manner of occurrence of the 

 bodies, or, in other Avords, the conditions under which they consolidated, 

 and only secondarily on their age; hence that the age of a rock can only be 

 relatively and not absolutely determined b}' its internal structure and petro- 

 graphical constitution. The details of the microscopical structure of the 

 various I'ock species are so fully described and discussed by Mr. Cross in 

 Appendix A that only a few of the more prominent characteristics of the 

 two types, such as will serve to correlate them with those of other regions, 

 need to be given here. The older series are either entirely granular, or, 

 where porphyritic, are characterized by the holocrystalline structure of 

 the groundmass and an absence of isotropic or amorphous material, when 

 examined under the microscope. Many of the orthoclastic varieties have 

 extremely large crystals of that feldspar, which give a striking and easily 

 recognizable appearance to the rock masses ; although very prominent 

 in Colorado, this peculiarity can hardly be regarded as an essential charac- 

 teristic of the type. They are not vesicular or scoriaceous; in other words, 

 they present the external characteristics of a rock cooled under pressure. 

 The younger type, however, while in exceptional instances almost holo- 

 crystalline, generally contains isotropic material or actual glass substance. 

 Its orthoclastic feldspars are essentially sanidine, it may be vesicular and 

 scoriaceous, and in general carries abundant glass inclusions and bears evi- 

 dence, either in its structure or in the constitution of its mineral constituents, 

 of having cooled at or near the surface, and consequently more rapidly than 

 the older type. In the Mosquito region there is apparently a definite rela- 

 tion between age and relatively granular character of the different varieties 

 of either type; thus White Porphyry is the most thoroughly granular rock 

 among the older series, and Nevadite among the younger. Although the 

 relation of pressure and conditions of cooling to internal structure are so 

 marked and important in the two great series, or, so to speak, generically, 



