18 DESCRIPTIVE GEOLOGY. 



erable distances. They also occur in the granitoid rocks. Medium-sized, 

 irregular crystals of dark iron-garnets were observed in the coarse granite, 

 while small red garnets occur in the gneissic strata near Signal Peak, and 

 again near the head of the Sybille Cailon ; both localities in the immediate 

 neighborhood of observed hornblendic gneisses. Iron-pyrites occur, as 

 already mentioned, in the graphite beds, and were observed in a narrow 

 vein associated with quartz and the iron minerals. Although but little has 

 been accomplished in the careful study of the field, it may be stated that 

 the Archaean series of the Laramie Hills affords no very great variety in 

 mineral species as compared with many localities of the same age. It may 

 be well, however, to give, in a concise form, the species observed. The fol- 

 lowing are the essential minerals of the larger rock masses : Quartz, ortho- 

 clase, oligoclase, labradorite, biotite, muscovite, chlorite, hornblende, and 

 diallage. As accessory minerals, there occur magnetite, ilmenite, graphite, 

 pyrites, garnet, and epidote. Under the microscope were detected lepi- 

 domelane, zircon, and apatite. Large bodies of eruptive granites, such as 

 have been observed in the Colorado Range to the south, probably do not 

 occur in the Laramie Hills ; indeed, the presence of eruptive rocks, younger 

 than the Archsean, such as the porphyries, diabases, and diorites, were nowhere 

 observed, and, if present, will most likely be found in the form of narrow 

 dikes. 



This is also true of the more recent volcanic rocks. Indeed, from the 

 Laramie River southward, none of the volcanic rocks occur, either in the 

 mountains or along the east flank of the range, within the limits of the 

 survey. The first outbursts seen going southward are the well-known dikes 

 of basalt at Valmont on Boulder Creek in Colorado, which break through 

 the Cretaceous beds. At a short distance farther to the south occur the 

 horizontal basaltic beds, capping the same strata near Golden City. 



Physical Description of the Colorado Aech^an Body. — But a small 

 portion of the Colorado Range south of the Laramie Hills comes within 

 the explorations of this survey, the soutliern boundary of the field-work 

 not extending beyond the region of Long's Peak, about the line of 40° 

 15' of north latitude. It embraces then only about 30 miles in a north 



