PRELIMINARY HANDBOOK OF THE DEPARTMENT OF GEOLOGY. 29 



account of the largo scale on which the work has been carried out be 

 showu only in models and illustrations. Gilbert's stereogram of the 

 Henry Mountains is one of the most striking objects now in this series, 

 though attention should also be called to Diller's models of Mount 

 Shasta, California; Becker's models of the Washoe district, Nevada; 

 Dutton's models of Mount Taylor, New Mexico, and the High Plateaus 

 of Utah. 



(G) Veins. — Professor Geikie treats the subject of veins and vein 

 formation uuder three heads: (1) Mineral veins, (2) Eruptive veins or 

 dikes, and (3) Segregation veins. 



For the present the known eruptive veins in the collections are 

 grouped with the nou- volcanic igneous ejections as dike rocks, and here 

 we have to do with only the first and third of the above divisions. The 

 term vein is used by the above authority to designate "any mass of 

 mineral matter which has solidified between the walls of a fissure. When 

 this mineral matter has been deposited from aqueous solution or from 

 sublimation, it forms what is known as a mineral vein. When it has 

 crystallized or segregated out of the component materials of some still 

 unconsolidated, colloid, or pasty rock, it is called a segregation vein." 



Simple as such a division may seem it is not always easy, or indeed 

 possible to ascertain from a simple examination of the specimens to 

 which of the two groups they may belong. As a rule the mineral veins, 

 which appear to correspond to the fissure veins of other authors, are 

 separated by sharp and well defined walls from the couutry rock, and 

 may, and often do, show a well defined banded or comb structure as 

 shown in the quartz and rhodochrosite vein from the silver mines at 

 Butte, Montana (3856G), and less distinctly in that composed of ruby 

 silver and other silver sulphurets together with rhodochrosite from the 

 Reese River district, Nevada (15136). The segregation type is less dis- 

 tinctly marked, the vein material being welded to the inclosing rock 

 owing to the mutual protusion of* the component materials. This type 

 of vein is quite common in granitic rocks and is wellshown in the large 

 specimens from Rockport, Massachusetts (38757) and Auburn, Maine 

 (39057 and 39058). 



IV.— STRATIGRAPHICAL OR HISTORICAL GEOLOGY. 



Under this, the fourth subdivision, is considered the chronological 

 succession of the geological formations, the rocks being arranged ac- 

 cording to the order of their deposition or ejection. From this series 

 fossil forms will to a considerable extent be excluded as belonging 

 more properly to the department of paleontology. Only a few of the 

 more characteristic forms from each horizon will be shown. 



As at present contemplated the main idea is to show that the same 

 geological forces have been in operation and rocks of the same general 

 nature been in process of foimation from the earliest time down to the 

 H. Mis, 224, pt. 2 58 



