abstracts: geology 529 



basins, owe their present bed rock configuration to the erosion effected 

 by long valley glacier ice streams that had their sources on the Arctic 

 divide and flowed southward. Upon the retreat of these valley glaciers, 

 which have now entirely disappeared from this part of the Arctic 

 Mountains, widespread deposits of glacial outwash gravels, sands, and 

 silts were left along the large valley. The present large streams have 

 dissected and aggraded their flood plains but large quantities of the 

 older gravels still remain as wide sloping terraces. In many cases the 

 gold-bearing gravels represent old pre-glacial stream gravels. These 

 deposits are now buried under silts and recent stream gravels and are 

 mined by shafts and drifts. But most of the gold production has been 

 from shallow deposits of gravel along the present streams. A. G. M. 



GEOLOGY. — -Ore deposits of the Helena mining region, Montana. 



Adolph Knopf. Bulletin U. S. Geological Survey No. 527. Pp. 



143, with maps, sections, and illustrations. 1913. 

 The Helena mining region is an area of 1300 square miles in south- 

 western Montana. The oldest rocks consist principally of sediments 

 ranging in age from Algonkian to Cretaceous. They include mainly 

 limestone, shale, and quartzite and lie in angular accordance from the 

 lowermost member to the top of the series. They are conformably 

 overlain by andesite and latite lavas and breccias of probable late Cre- 

 taceous age. These older rocks were invaded by a large granite mass 

 which forms the northern extension of a great intrusion in southwestern 

 IVIontana, known as the Boulder batholith. Large intrusions of aplite 

 in irregular masses and dikes followed the main irruption. They are 

 commonly tourmaHniferous, and in places, notably so. In late Miocene 

 time, a series of dacites, consisting of lavas, tuffs and breccias, locally 

 at least 2400 feet thick, were extravasated upon the deeply eroded sur- 

 face of the granite and older rocks. 



The ore deposits of the region fall into two distinct groups, widely 

 separated in time of origin. The older are late Cretaceous or early 

 Tertiary in age, the younger are post-Miocene. The ore bodies of the 

 first period of mineralization are mainly argentiferous lead and gold- 

 silver deposits. They have furnished the greater part of the production 

 of the region; in fact, the value of their output has been roughly three 

 times that of the post-Miocene deposits. The argentiferous lead de- 

 posits constitute the prevailing type of ore body of the older group. 

 They are situated as a rule near the contact of the granite and the rocks 

 invaded by it, and are replacement-fissure lodes containing galena, sphal- 



