62 BASIC AND ULTRABASIC IGNEOUS ROCKS-BENSON. tMEMOm 1vo A L T .xix; 



diorite-batholiths formed, and these in turn were dislocated by Laramide orogeny. Intrusions 

 of nepheline-syenite and its associated differentiates followed, and were succeeded by sodic 

 hornblende-gabbros of Tertiary age and later granites of a more acid character. The conclud- 

 ing events were the intrusion of dikes of basalt and andesite. Daly ('12) has offered an explana- 

 tion of this series in terms of the theory of assimilation, in which the nepheline-syenites are 

 looked upon as a peculiar though initial member of a second group of chiefly granitic intru- 

 sions, derived in large measure by the further differentiation of the refused granodiorite which 

 formed the latest member of the earlier series. The discussion of this suggestion is however, 

 outside the scope of this paper. 



Southward in the State of Washington (Mount Stuart and Snoqualmie quadrangles) ser- 

 pentine occurs intrusive into what is probably a Paleozoic series, and is overlain by Eocene 

 sediments, in which serpentine pebbles occur. Unconformably upon these, are Miocene basalts 

 and andesites, and the whole sequence has been invaded by granodiorite (Smith '04, Smith 

 and Calkins '06) . 



Southward from here we may conveniently recognize two main zones of intrusions of 

 ultrabasic and basic rocks, namely, those in the Sierra Nevada and those in the Coast Ranges, 

 though these are much the same in age. These have been studied in considerable detail, and 

 an excellent series of maps are included in the folios of the Geological Atlas of the United States. 

 The more easterly series passes through the following quadrangles: Bidwell Bar, Downieville, 

 Smartsville, Colfax, Nevada City, Truckee, Sacramento, Placerville, Pyramid Peak, Jack- 

 sonville, Big Trees, and Sonora, which have been mapped and described by Becker, Lindgren, 

 Ransome, and Turner. The main formations throughout this region are a series of highly 

 altered Paleozoic rocks, the Calaveras formation, on which lies the less-altered but still highly 

 crushed Trias-Jura Mariposa formation, which is largely slaty, containing intercalated basic 

 lavas and breccias. Into this have been injected long bands of serpentine and gabbro, which 

 in some areas have irregular transgressive boundaries, but in others, especially in the Sonora 

 quadrangle (fig. 18), the boundaries are very concordant with the strike of the invaded forma- 

 tions. Dioritic and granitic intrusions accompanied these invasions, but the sequence of the 

 intrusive masses is not always clear. Dioritic and diabasic dikes frequently occur in the ser- 

 pentines. The age of these intrusions is fixed by the fact that in the Truckee quadrangle they 

 are overlain by Upper Cretaceous (Chico) sediments. An interesting feature is the presence of 

 albitic dike-rocks associated with the serpentine in the Bidwell Bar, Jacksonville, and Sonora 

 districts. These are nearly white rocks, with minute faint blue mottlings. The chief con- 

 stituent is albite, and the mottling is due to the presence of tufts of slender crystals of amphi- 

 boles, with a little aegyrine, apatite, and zircon. Biotite is present in one of these dikes. Some- 

 times the dike is a type of soda-granite, containing albite, quartz, muscovite, and a little sphene. 

 The largest of these masses has a length of 6 miles and a width varying up to 100 yards. It 

 was injected between the serpentine and the inclosing hornblende-schist. Though such rocks 

 seem rare, the writer has described several occurrences in association with serpentine from 

 New South Wales and has pointed out other recorded instances in Cornwall and Akmolinsk 

 (Benson '13, '18, '18a). They seem to be formed according to some general scheme of para- 

 genesis. (Cf. Dresser ('21) and the footnote on p. 32.) 



Structurally the Klamath Mountains of Oregon should be grouped with the Sierra Nevada, 

 but as they form a link between the latter and the Coast Range series of ultrabasic intrusions 

 they are considered here. Detailed investigations have been made by Diller in mapping the 

 Roseburg ('98), Redding ('06), Coos Bay ('01), and Port Orford ('03) quadrangles; while the 

 geology of the whole region has been more recently summarized by Diller ('14), and Smith and 

 Packard ('19). In the Devonian and Carboniferous periods there was a transgressing sea of 

 varying depth, in which were deposited about 10,000 feet thick of argillites, tuffs, and sand- 

 stones, with lenses of limestone and frequent beds of radiolarian chert. With these are asso- 

 ciated an abundance of andesitic lavas, rich in pyroxene. In some places they are vesicular, 

 and associated with fragmental deposits due to explosive volcanic action. These were folded 

 and uplifted during the close of the Paleozoic period, and were invaded by peridotite, gabbro, 



