ON STRUCTURAL FEATURES OF MINERAL VEINS. 37 



resemble rounded, water-worn boulders, a similarity which has 

 suggested some rather improbable hypotheses of deposition. 



1.05.03. Clay Selvage. An extremely common feature is a 

 band of clay, most often between the vein matter and the wall. 

 This is called a selvage, gouge, flucan, clay seam, or parting. It 

 may come in also between layers of different minerals, and even 

 rests as a mantle on the crystals which line cavities. It is at times 

 the less soluble portion left by the decay and removal in solution 

 of wall rock (residual clay), at times the comminuted material re- 

 sulting from the friction of moving walls (attrition clay), and 

 again it may be taken up by currents and redeposited from bodies 

 of the first two sorts. Such layers of clay, being wellnigh imper- 

 vious to water, may have exercised an important influence in di- 

 recting the subterranean circulations. (See Becker on the Corn- 

 stock Lode, 2.11.19.) 



1.05.04. Pinches, Swells, and Lateral Enrichments. The 

 swells and pinches of veins have been referred to above and ex- 

 plained. Aside from these thicker portions of the ore, it is often 

 seen that the richer or even the workable bodies follow certain 

 more or less regular directions, forming so-called " chutes." They 

 probably correspond to the courses taken and followed by the 

 richer solutions. J. E. Clayton observed that they follow the di- 

 rections of the slips, or striae, of the walls rather more often than 

 not, and in the west this disposition or tendency is called Clayton's 

 law. Chute is sometimes spelled "shoot " or "shute." Chimney 

 and ore-cource are synonyms of chute. Bonanza is used, especially 

 on the Comstock Lode, to indicate a localized, rich body of ore. 



Lateral enrichments are caused by the spreading of the ore- 

 bearing currents sidewise from the vein, and often along particu- 

 lar beds of rock, which they may replace more or less with ore. 

 Beds of limestone it may be quite thin, when in a series com- 

 posed of shales or sandstones are favorite precipitants, and from 

 such lateral enlargement the best returns may be obtained. The 

 valuable ore bodies of Newman Hill, near Rico, Colo., whose inter- 

 esting description by J. B. Parish has already been several times 

 cited, are found as lateral enrichments along a bed of limestone less 

 than three feet thick and embedded in shales. Above the lime- 

 stone the veins practically cease. Lateral enrichments may closely 

 resemble bedded deposits if the supply fissures are relatively small. 

 This interpretation is placed by W. P. Jenney on the disseminated 

 lead ores of southeastern Missouri (2.15.09), and he has suggested 



