518 proceedings: geological society 



positions for a period of at least about a century and a half. It may 

 be mentioned that the crater of Stromboli is an exceptionally favorable 

 one for the study of such a feature, as it is bounded on two sides by 

 prominent ridges, one or both of which appear in all views and plans, 

 that form permanent landmarks by which the relative positions of 

 the several vents at different dates can be readily established. A 

 series of about 15 of these views and plans, dating back to 1768, was 

 shown. 



This feature of Stromboli, and possibly of other volcanoes, as it 

 seems to be presented at Kilauea and elsewhere, does not appear to 

 have been generally recognized. It would, however, seem to have an 

 important bearing on certain volcanological problems, such as, among 

 others, the size of the lava reservoir immediately beneath the crater 

 floor. Such a persistence of location of volcanic vents apparently 

 favors Daly's view that the size of volcanic conduits is small, rather 

 than Dana's, that they are nearly commensurate in size with the whole 

 crater floor. The formation of these relatively small vents may possi- 

 bly be explained by Daly's "gas-fluxing" hypothesis. 



F. C. Schrader: Ore deposits of the Rochester district, Nevada. The 

 deposits consist of silver and gold-bearing veins, lodes, and associated 

 replacement bodies. They occur in volcanic rocks which are chiefly 

 rhyolites of Triassic age. The rocks, nearly 2,000 feet in thickness, 

 dip gently to the east. The veins dip steeply to the west. In some 

 of them good ore bodies are opened to the depth of a thousand feet. 



The deposits lie in two north-south belts, Henzel Hill belt on the 

 east and Lincoln Hill belt on the west, which are about two miles 

 apart, each a mile wide and five miles long. In the Henzel Hill belt 

 they are chiefly silver-bearing, in the Lincoln Hill belt gold-bearing. 



Henzel Hill belt near its middle point contains Henzel Hill, an oval 

 silicified knob 3000 feet long, the seat of the most important deposits. 

 Here the deposits occur in and associated with fissures, joint planes, 

 and shear zones. Some of them are 40 feet in width. The ores which 

 average about $20 to the ton contain chiefly silver but carry also 

 several dollars to the ton in gold, which increases in amount with 

 depth. 



The ore minerals are chiefly argentite and sulphantimonites with a 

 little associated proustite, cerargyrite, bromyrite, pyrargyrite, scales of 

 native silver, and specks of free gold. From the 200-foot level down 

 the ore minerals are mostly sulphides. 



At Packard, 2 miles south of Henzel Hill, the deposits occur as massive 

 replacement ore beds, nearly 100 feet in maximum width, in soft schis- 

 tose rhyolite. They contain but little quartz. The ore minerals are 

 chiefly cerargyrite and argentite. Most of the ore produced up to 1916 

 averaged in silver about $50 to the ton. 



In the Lincoln Hill belt the deposits are more distinctly narrow veins 

 of the filled fissure type. The gangue is quartz which contains almost 

 exclusively free gold ores, averaging about $140 to the ton, with some 

 that are very rich. The associated minerals are pyrite, arsenopyrite, 



