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Gold has been discovered recently at Cortez, Nevada, about 45 miles southwest of Carlin in carbonate rocks in a window of the Roberts Mountains thrust. The host rock consists of laminated to thin-bedded dark-to light-gray, silty dolomitic limestone and calcareous dolomitic siltstone in the upper part of the Silurian Roberts Mountains Limestone. These rocks contain sparse pyrite cubes and aggregates and some organic carbon. The rocks have been faulted and folded repeatedly during their complex geologic history. The gold is disseminated in a large zone where the rocks have been fractured and bleached and the pyrite oxidized. During oxidation the iron was redistributed, giving the rock a color ranging from light gray to dark red. The alteration zone envelops a 34-m.y.-old intrusive body of biotite-quartz-sanidine porphyry, which is also altered. No genetic relationship between the mineralization and the intrusive body is known. Silicification, iron-oxide staining, decalcification and, in extreme cases, dedolomitization generally accompanied the gold metallization, although any one of these phases of hydrothermal alteration may have been well developed without introduction of significant amounts of gold. Some clay alteration occurred in the igneous rock but none in the ore body. The gold is in micron-sized particles of native gold. Gold is taostly with silica between original silt grains and to a lesser extent in quartz-filled microfractures and hematite-goethite pseudomorphs after pyrite. The gold was discovered during the examination of an arsenic-antimony-tungstenmercury geochemical anomaly known in the area. Other gold deposits in north-central Nevada are associated with such anomalies. Contents

In connection with a symposium on core drills, it may be of some value tooutline briefly the evolution of the double barrel core drill. The single barrel core drill, or Texas barrel, has been described a numberof times and is undoubtedly merely an evolution of the fishing tool known as abasket. The single barrel drill has served its purpose, its use is decliningand it will not be further discussed in this paper. Jan Koster appears to have been the inventor of the double barrel core drillfor use in loosely consolidated formations. So far as can be learned at thisdate his invention was necessitated by the desire of the Holland GeologicalSurvey to obtain cores in localities where the diamond drill was unsuccessfulby reason of the unconsolidated nature of the formations to be penetrated. Inthe "Memoirs of the Government Institute for the Geological Exploration ofthe Netherlands," W. A. J. M. van Waterschoot van der Gracht says,"this device (the Koster drill) works admirably well in the boring atBaarlo; we succeeded in obtaining a section of many hundred meters of cores, out of sandy clays and scarcely coherent sands." No attempt to introduce the Koster drill into the oil fields of this countrywas made until 1919, when the Shell Co. of California obtained one and testedit in shallow exploration work in an area near what is now the Santa Fe Springsoil fields. The Koster drill appeared to be a step in the right direction, although itsperformance was not satisfactory, due chiefly to inadequate surface equipment.The cores obtained with it were usually short, because the opening to the innerbarrel usually became plugged before the barrel was filled with core. The ShellCo. of California made several changes in the design of the barrel whileoperating in the area mentioned above, and later at Coalinga, but apparentlywithout obtaining complete satisfaction from the drill, as a punch core drillwas used by that company during the intensive drilling campaign at Long Beachin 1921. The original Elliott core drill, as used by the Elliott Core Drilling Co.from 1921 to 1924, was somewhat similar in design to the Koster drill, butadapted to the taking of cores at considerable depth and with several changesmade necessary by California practice in rotary drilling and Californiaformations.

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