280 W. Lindgren — Auriferous Conglomerate, etc. 



mortar and washed in a miner's pan ; in all instances gold was 

 obtained ; the particles — colors — were very fine and flaky but 

 concentrated easily ; sometimes only one or two, sometimes a 

 great number of colors were obtained. Beside the gold there 

 was invariably some iron pyrites nearly always found in con- 

 glomerates and as well known resulting from the reducing action 

 of organic substance on circulating iron salts. The absence of 

 magnetic and titanic iron ores — black sand — was very noticeable. 

 In order to obtain a better idea of the amount contained Mr. 

 G-. F. Deetken kindly made duplicate assays for me of speci- 

 mens from the two shafts with the result that the conglomerate 

 from the south shaft was found to contain in average 36 ^ pr. 

 ton in gold while a distinct trace was obtained from that from 

 the north shaft. 



From the general character of the occurrence I believe 

 it represents shore gravels accumulated near the mouth 

 .of some river draining the Carboniferous area and empty- 

 ing into the muddy gulf in which the Mariposa slates were 

 deposited. It is not likely that the deposit will ever be 

 of economic importance though there is a possibility that by 

 thorough prospecting and cross cutting the, strata a richer bed 

 might be exposed. 

 Conclusions : 



(1) The Mariposa beds were deposited unconformably upon 

 the rocks of the Calaveras formation, then a land area of con- 

 siderable extent. From the abundant presence of phthanite 

 pebbles in the conglomerate it is certain that this older series 

 was already at that time considerably altered though the large 

 quantity of limestone also contained makes it probable that the 

 metamorphic action has progressed much further in it since 

 that time. From the absence of igneous rocks in the con- 

 glomerate, accentuated by the absence of " black sand" in it,Jt 

 may be concluded that the numerous basic igneous rocks now con- 

 tained in the Calaveras formation in this vicinity have princi- 

 pally been intruded after the deposition of the Mariposa beds. 



(2) The old land area at that time already contained 

 auriferous quartz veins, the detritus of which enter into the 

 composition of the Mariposa conglomerate. 



(3) There are in the Sierra Nevada primary auriferous 

 deposits of very different age. The oldest antedate the Mari- 

 posa beds ; the greatest number were formed at the close of 

 the eruptive activity following soon after the deposition and 

 upheaval of the Mariposa beds ; the most recent, found along 

 the summit and along the eastern slope of the range were 

 formed towards the close of the eruptive activity in late Ter- 

 tiary time. 



