338 DYNAMICAL GEOLOGY. 



and spongy layers as represented in Fig. 313, and the thin harder bands in 

 this lamination or straticnlation are persistent throughout the lithophysae ; 

 as the figure shows they were sometimes arched in the making of the cavities, 

 while often, on the other hand, they prevented the cavity from completing a 

 circular form. The concentric partitions are fragile and consist mostly of 

 minute crystals of quartz, feldspar, and tridymite ; and sometimes topaz and 

 garnets are in the cavities. 



Richthofen regarded the lithophysae as made by expanding steam, like vesicles in 

 ordinary lava, and the concentric partitions as having been thrown off in the progress of 

 the expansion, and hence the name. Mr. Iddings points out close relation betvvreen 

 the lithophysae and the associated radiate spherulites, and doubts the vesicular mode 

 of origin. The following is a possible explanation. If the cavity made by vesiculation 

 became at first filled with an aqaeo-igneous or jelly-like solution of the rock, the concentric 

 shells may be a centripetal result, due to progress in cooling, and loss of moisture from the 

 outside. The process would first produce a deposition of crystals over the confines 

 or wall of the cavity, and thus deprive the inside solution, adjoining this wall, 

 of part of its mineral material ; then, the succession of shells might form inside in a 

 manner analogous to that given for concentric rings on page 130. Johnston Lavis regards 

 lithophysae as concretions growing radiately outward, and refers the spaces between the 

 concentric shells to the liberation of vapor from moisture contained in the glass, this 

 liberation taking place as the glass becomes changed to feldspar in solidification. Whitman 

 Cross, who adopts the vesiculation theory, found beautiful but minute crystals of topaz 

 and garnets in lithophysae of the rhyolyte, of Nathiop, Col. (1884, 1886). Iddings and 

 S. L. Penfield have described (1885) yellow crystals of fayalite from those of the black 

 obsidian at Yellowstone Park. Utah rhyolyte also has afforded topazes. 



Veins made by the aid of deep-seated Igneous Ejections. 



For the formation of veins through the heat of igneous ejections, the 

 earth's crustal heat has been the agent, aided possibly by heat from local 

 crushing and friction. The fissures at great depths may have had the heat 

 required, without addition from mountain-making movements. The general 

 steps of progress — that is, the methods of transfer and formation of mineral 

 material by heated vapors — are the same that have been described. 



Fissures descending to regions of fusion are necessarily deep fissures, 

 and for this reason the veins that have been made in connection with them 

 include the richest of ore-bearing veins. The deep fissures let out liquid 

 rock. But they were the means of opening a way for whatever vapors or 

 solutions the melted rock through its heat, supplemented by the earth's 

 crustal heat, might gather from the rocks, or their crevices, along the way 

 up, or from the depths below. The copper veins of the Lake Superior 

 region are an example ; and so are also the richest and the chief part of all 

 the silver, lead, and copper veins of western America, from Fuegia on the 

 south, along the western slope of the Andes to Central America, Mexico, 

 Nevada, Arizona, Colorado, Utah, and Wyoming. 



The results differ not only according to the kinds of rocks below, but 

 also the kinds along the upper part of the fissure : whether they are (1) of dif- 

 ficult corrosion, or (2) of easy corrosion like limestones. 



