22 abstracts: geology 



Washington, containing within its boundaries the whole of Stevens and 

 Ferry counties. 



The rocks of this region are referred to the Proterozoic, Paleozoic, 

 Mesozoic, and Cenozoic eras. The rocks classified provisionally as 

 pre-Cambrian are the crystalline schists and metamorphosed limestones 

 and quartzites found between Laurier and Orient. The larger part of 

 the region is occupied by an extensive series of more or less dynamo- 

 metamorphosed argillites, lime shales, limestones, and quartzites. Be- 

 cause of its general similarity to rocks farther west in Washington, 

 which in places are fossiliferous, this series has been referred to the 

 Paleozoic era. In places these rocks were intruded by diabase or it 

 flowed out over their surfaces and was metamorphosed along with the 

 rest of the series, so that it is now represented by greenstone. In part 

 of the district greenstones are prominent. One of the most conspicu- 

 ous rocks of northeastern Washington is intrusive granite. It is thought 

 that large batholiths intruded the sedimentary series during Mesozoic 

 time, parts of the batholiths having been subsequently exposed by 

 erosion. The presence of monzonite porphyry in most of the mining 

 districts is believed to be due to later eruptions of the granitic magma, 

 and these may have taken place at intervals from late Mesozoic to 

 early Tertiary time. Lava flows of various types fill many of the 

 erosion depressions in the older rocks, and in places form high moun- 

 tains. Their eruption is thought to have taken place during Tertiary 

 time. Still later flows of basalt are prominent in the extreme southern 

 part of the area investigated, although some were seen in the Republic 

 district. The gold-quartz veins of the Republic (Eureka) district and 

 the First Thought gold deposits, in the Orient district, are of late Ter- 

 tiary age and are seliniferous. 



The period in which the granite, quartz diorite, quartz monzonite 

 porphyry, and monzonite porphyry and the accompanying dikes were 

 intruded was that of the most extensive mineralization. The major 

 part of the ore deposits are believed to be of Mesozoic age, and they 

 owe their origin to the after effects of igneous intrusion. During the 

 Paleozoic period of dynamometamorphism, while the greenstones were 

 being formed, some of the metallic ores may have been concentrated 

 from the basic rocks, so that a few of the ore deposits in the area may 

 be of Paleozoic age. However, intrusive rocks thought to be of Meso- 

 zoic age were found in the vicinity of the greenstones, and it is more 

 likely that the deposits associated with the greenstones are due to the 

 Mesozoic intrusions. Alfred H. Brooks. 



