GEOLOGIC RELATIONS AND ACJE. 225 



deposits. In the larger part of tlie gold region a wide belt of Paleozoic 

 slates ooni])aratively poor in gold deposits separates tliis contact from the 

 principal g()ld-i)roducing districts. In very many places, however, the 

 contact clearly marks the abrupt beginning of auriferous deposits, though 

 perhaps ])oor and of small extent. The sudden change of recent and 

 Tertiary river-beds from barren to auriferous when cutting across the 

 contact is often very noticeable. 



Though not ai)plicable to the main granitic contact, the statement 

 quoted is to a certain degree true of the smaller masses of granodiorite 

 scattered through the metamorj)hic series, for it is very common to find 

 the gold-quartz veins clustered near their contacts in the manner indi- 

 cated. It is not so general, however, as to be called a rule or a law, for 

 there are many included granitic masses the contacts of which are in no 

 way remarkable for alnindant deposits. 



Dr W. >[oericke, who has recently published several very interesting 

 pa])ers on the gold deposits of Chile, has come to tlie conclusion that they 

 are closely associated with acid, igneous rocks, and drawn a comj)arison 

 between the occurrences of that country and California.* In view of this, 

 it may be well to em])hasize the fiict that the gold-(|uartz veins of Cali- 

 fornia do not in their surface relation show any remarkable dependence 

 on acid, igneous rocks. The great mother-lode, for instance, is in loca- 

 tion and occurrence of its ores in no way related to such rocks, they 

 being, on the contrary, as a rule, distant from it. 



Normal gold-quartz veins in diabase and augite-porphyrite sometimes 

 occur far away from other rocks, although the larger areas of the former 

 are, on the whole, rather barren. 



Age. 



l>('fure beginning the discussion of the characteristics of the deposits, 

 tln.'ir age may be briefly touched upon. It has long been ai)j)arent and 

 insisted upon by Whitney, von Richthofen and others that the (piartz- 

 veins of California are of late Jurassic or early Cretaceous age, and the 

 same authors have suggested that they probably owe their origin to 

 thermal action folhjwing the granitic intrusion. For the larger nundier 

 of the ([uartz-veins this is undoubtedly true. It is certain that the 

 majority were formed subsequent to the latest dynamo-metamorphism 

 of the sedimentary and old eruptive rocks of the Sierra Nevada, subse- 

 quent also to the granitic intrusion. It is, however, also certain that 

 some deposits antedate this period, for in the latest sedimentary member 

 of the l)ed-ror'k series there are conglomerates containing quartz pebbles 

 and free gold, t which appears to have been concentrated as placer gold 



•Zeitfluhrift fur prakt. Geol. Jahrgang, 1804, p. 28. 

 t W. LindKien : Am. .Jour. Sci., October. 1894. 



