390 H. W. TURNER 



cases being open. Other specimens of a fine-grained red sili- 

 ceous rock which forms part of the lode, appear to represent a 

 calcareous shale subsequently silicified. The red rock contains 

 very abundant bodies, smaller than the Loftusia determined as 

 such, but probably also of foraminiferal origin. Whatever the 

 original nature of these smaller tests, they are now composed of 

 granular quartz like the Loftusia. There is here unequivocal 

 evidence that a considerable mass of carbonates has been 

 replaced by silica. A large portion of the vein material contain- 

 ing the precious metals of the Diadem lode is chalcedony. 

 The same waters which deposited the quartz and chalcedony and 

 replaced the carbonates of the foraminifera tests are without 

 doubt responsible for the gold, silver, manganese, etc., found in 

 the deposit. This vein deposit may therefore be called a replace- 

 ment deposit. The Diadem lode lies in the fault zone along 

 which displacements have formed the steep slope east of Spanish 

 Peak. There is also evidence of faulting in comparatively recent 

 times at the lode itself. 1 It is without doubt along these faults 

 that the waters containing the silica, etc., of the deposit have 

 found their way from below. Such being the case it is likely 

 that a certain portion of the secondary material may represent a 

 true vein deposit, but it is probable that the larger part of the 

 lode, which is represented by Edman as being in places sixty 

 feet wide, may be called a replacement. 



Professer Whitney inclined to the belief that the great quartz 

 veins of the mother lode represent the replacement of bodies of 

 dolomite. As Lindgren remarks, 2 however, this theory has not 

 been supported by more detailed investigation. H. W. Fair- 

 banks has suggested that these very large veins of pure quartz, 

 sometimes forty feet in width, have resulted from the replace- 

 ment of dikes of basic igneous rocks. The mechanical difficulty 

 of accounting for the existence of such wide fissures, and the 

 fact that the vein matter in these large masses seldom shows a 

 banded structure such as might be expected from the deposit of 



1 Seventeenth Ann. Rep. U. S. Geol. Surv., Part I, p. 553. 



2 Bull. Geol. Soc. Am., Vol. VI, p. 235. 



