26 DESCRIPTIVE GEOLOGY. 



general strike being north and south, with a tendency to the west of north 

 and east of south. Those observed were chiefly feldspar- and quartz-por- 

 phyries, with both a coarse- and fine-grained groundmass. A typical feld- 

 spar-porphyry cuts the Archaean series, on the west side of Long's Peak. 

 It is made up of a compact crystalline groundmass of feldspar, quartz, and 

 hornblende, with all the constituent minerals present porphyritically en- 

 closed. Both orthoclase and plagioclase are present, the former, however, 

 most prominent. The hornblende occurs in short, fibrous, green prisms, 

 while the quartz is in small, round grains.- As an accidental mineral, a 

 small quantity of white mica may be detected associated with quartz. Un- 

 der the microscope, liquid-inclusions are seen, both in the quartz and ortho- 

 clase. Professor Zirkel has called attention to the differences in form be- 

 tween the inclusions in the quartz and those of the feldspar ; while in the 

 former they are chiefly globular, in the latter they are very irregular in out- 

 line. Other dikes occur, darker in color, and more basic, doubtless from the 

 greater amount of hornblende present in the groundmass. The occurrence 

 of hornblende, abundant in these intrusive porphyries, is of interest, as it 

 appears to be an exceptional mineral in the Archaean beds here. Narrow 

 veins of pure quartz traverse the range in a number of localities, but, so far 

 as known, none of any economic value have as yet been found, although so 

 abundant and rich in mineral species in Central Colorado. Dark iron-gar- 

 nets occur somewhat sparingly, disseminated in the more schistose beds. 

 They were observed on the upper part of Fish Creek in mica-bearing 

 strata, and again near the mouth of Dale Creek. Magnetite, in small 

 iiTegular particles, was seen in a number of localities ; while it probably 

 may be found scattered through all the beds,' it was observed more fre- 

 quently in the coarser-grained granites. 



Eastern Foot-hills. — The sedimentary rocks which form the out- 

 lying ridges and foot-hills of the east side of the Colorado Range are made up 

 of- Palaeozoic and Mesozoic strata, containing both sandstone and limestone 

 beds. They form a very marked physical and geological feature, a narrow 

 border region, lying between the massive Archaean rocks on the one side, 

 and the broad level j)lains of the Upper Cretaceous series of Colorado and 

 the nearly horizontal Pliocene Tertiary series of Wyoming on the other. 



