352 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



M 10-11. WASHINGTON AND BBITISH COLUMBIA. 



Umpleby *^^ gives the following account of the geology of the Republic dis- 

 trict, Washington, which may be regarded as representative of the Paleozoic and 

 later rocks mapped as metamorphic Paleozoic in this region. 



The geologic history of the Repubhc district has as its great features sedimentation in the 

 Paleozoic, erosion with minor volcanism in the Mesozoic, igneous activity and erosion in the 

 Tertiary, and continental glaciation in the Quaternary. 



The oldest rocks exposed in the district are the metamorphic equivalents of a great series 

 of shales, sandstones, limestones, and lava flows which are of Paleozoic age and are provisionally 

 assigned to the Carboniferous. After the deposition of this series the area passed through a 

 long period of crustal disturbance which, although not developing sharp folds, metamorphosed 

 the beds and raised the area far above sea level. Either during this period of crustal disturbance 

 or shortly thereafter great bathohthic masses of granodiorite were intruded into the Paleozoic 

 series. 



From the time of the granodiorite intrusions, which are probably of early or middle 

 Mesozoic age, to middle Tertiary times there was a great period of erosion which may be 

 divided into two parts — a first, during which the entire area was reduced probably to base 

 level (Eocene surface); and a second which was introduced by decided elevation and during 

 which broad valleys at least 2,500 feet deep were developed. 



The next rocks in order of formation are of Oligocene age and occupy one of these broad, 

 deep valleys. They are dacite flows, including great quantities of stream gravels. Overlying 

 these, unconformably, are andesite breccias, lake beds, and andesite flows, aU of which occur 

 within the old erosion vaUey. Next in order of age are intrusive latite porphyries with which 

 the ore deposits are thought to be genetically related. 



From the time of the latitic intrusions to the Pleistocene erosion was the dominant process, 

 although during this time there was a short period of basaltic eruption. In the Pleistocene 

 period the Cordillera ice sheet covered the entire area. 



The oldest known formation exposed in the area is a series of metamorphic rocks including 

 slates, schists, quartzites, marbles, and greenstones. It is most extensively developed south- 

 west of Repubhc, where about 2 square miles are known. East of the San Poil-Curlew valley 

 * * * are extensive areas where the metamorphic series is the predominating formation. 

 Both areas are so poor in exposures that it is impossible to determine detailed relations. 



The Paleozoic rocks are very uniformly but not intensely metamorphosed. True schists 

 are not common, and in many instances the hmestone has not been changed to marble. Never- 

 theless, the series has been so disturbed that a given set of characteristics seldom persists for 

 more than a short distance in any direction. Neither bottom nor top of the series was found. 



Black carbonaceous argiUite is the predominant rock type, although bluish-gray non- 

 fossiliferous limestones have a wide development. Massive gray quartzites were noted in one 

 exposure southwest of Republic. Porphyries of intermediate and basic composition are found 

 both as dikes and siLls, apparently intruded into the series before its metamorphism. The age 

 relations of the various phases of the series are not obvious from studies in the Republic area, 

 but to the north, at Phenix, British Columbia, LeRoy'^ reports a section including aU the 

 above types of rocks, which he divides into three parts, with an unconformity between the 

 upper two. His section places the argUlites in the upper part, separated from the limestones 

 and tuffs (no tufl^s of this age were noted at Republic) by a pronounced unconformity, while 

 the lower member is quartzite with intruded dikes and sills of basic porphyrites. The Paleo- 

 zoic beds are folded and metamorphosed and are in marked contrast with the overlying Tertiary 

 series, in which folding is less marked and the beds are not metamorphosed. 



It is not possible, on the strength of facts now known, to assign this formation to a definite 

 place in the Paleozoic series. Near Republic the formation carries certain fossils, not well 



a Bull. Canada Geol. Survey No. 1072, p. 65. 



