As the underground is inevitably becoming a more and more serious option for development, the complexities should be dealt with in terms of legislation, policies and management issues. This chapter describes how the subsurface is managed and regulated. It discusses three strongly related and inter-linked issues: legal aspects, policy, and management. Legal aspects ( Section 6.1 ) deals with (written) laws and regulations concerning the subsurface. Such laws are normally produced by a government, approved by a house of representatives and enforced by the public sector at national, provincial, or municipal levels. Legislation is rooted in history and culture and this may explain similarities among (groups of) nations, also when it concerns the subsurface.

Abstract Ernie Anderson, Lauren Wright, and Bennie Troxel were among the first to rec-ognize the coeval nature of magmatism and crustal extension in the Colorado River and Death Valley extensional terrains, respectively. The earliest magmatic rocks in the Colorado River extensional terrain include the 15.7 Ma Aztec Wash and 15.8-16.9 Ma Searchlight plutons as well as the 15.3-17.4 Ma Spirit Mountain batholith. The Aztec Wash and Searchlight plutons are well exposed, stratified plutons that show a similar range in composition from 45 to 75 wt% SiO 2 . Homogeneous granites that compose about one-third of each intrusion are nearly identical in texture, major- and trace-element geochemistry, and isotopic composition; mafic rocks that are present in both plutons document basaltic input into felsic magma chambers. Isotopic data suggest that mafic magmas were derived from enriched lithospheric mantle with minor crust-al contamination; the more felsic rocks crystallized from hybrid melts that represent mixtures of juvenile basaltic magma + crustal melt, or products of anatexis of ancient crust + young mafic magmas that underplate the crust during extension. Although emplacement generally proceeded peak east-west extension, the Aztec Wash pluton is in part coeval with extension in the Colorado River extensional terrain, as indicated by the uniform north-south orientation of late dikes and mafic injections into the magma chamber. The Spirit Mountain batholith is more homogeneous and felsic than the Aztec Wash and Searchlight plutons; granite composition varies from 66.6 to 76.4 wt% SiO 2 across the batholith. Geochemical and isotopic data suggest that granites within the Spirit Mountain batholith may be derived by the melting of K-feldspar-dominated crust that includes both ancient and juvenile rocks, or by other processes involving fresh mantle input and Proterozoic rocks deep in the Miocene crust. The oldest and youngest plutonic rocks coeval with crustal extension in the Death Valley extensional terrain are the 12.4 Ma granite of Kingston Peak and the 9.8 Ma Shoshone pluton, respectively. Both plutons are texturally zoned and are charac-terized by rapakivi textures. Chemical and isotopic data suggest that the granite of Kingston Peak represents a partial melt of Mesozoic plutonic rocks at mid-crustal levels; the juvenile isotopic composition of this granite as well as the Shoshone plu-ton probably reflects post-subduction hybridization of mantle-derived mafic magmas with the crustal melt. Keywords: magmatism, crustal extension, Colorado River, Death Valley.

Over recent decades, numerous studies have highlighted the importance of opal, chalcedony and quartz varieties, chiefly in volcanic, but also in metamorphic and sedimentary environments. The focus is to define accurately their structures, composition and properties, as well as to identify the factors controlling the formation and the ageing of different forms of silica. In the field of archaeological sciences efficient discriminants are the bases from which the origin and provenance of materials may be traced. Substantial efforts were made in the attempt to combine geochemical, mineralogical, petrographic and geological features with archaeological and archaeometric information. However the results show that data integration is complicated, and several unanswered questions remain. On the one hand, archaeological research has focused on technological and ethnographic aspects, mainly concerning use-wear and heat-treatment studies. Mineralogical characterization has often been limited to the identification of the material, frequently by Raman microspectroscopy alone. On the other hand, the Earth sciences have provided basic mineralogical, crystal-chemical and geological knowledge, but failed to provide a systematic data collection of sources and their geochemistry. As a consequence, large gaps persist in the identification of archaeological opals, chalcedonies and quartz varieties, and in the geographic mapping of possible sources. In this context, the present review aims to summarize the current academic debate on such issues, possibly to encourage further work in the field. After a brief introduction to terminology, the structure of opals, their colours and properties are discussed, followed by an introduction to silica dissolution/precipitation and opal-formation processes. The next section reviews the information available on use of opals and provenance from historical sources, mainly Pliny the Elder, followed by a short list of ancient and modern opal supply areas, together with a (necessarily incomplete) summary of the geological and geochemical information. The discussion then encompasses chalcedony, agate and chalcedony varieties (carnelian, sard, onyx, sardonyx, chrysoprase, Cr-chalcedony, ‘gem silica’ or ‘chrysocolla chalcedony’ and heliotrope), following the same scheme as was adopted for opals. Terminology, distinguishing features, formation conditions, information derived from Pliny’s books, past and current supply areas and, finally, archaeometric provenance issues are addressed for each type of material. As for chalcedony, a comprehensive note on moganite has been included. The next section focuses on chert, flint and jasper. Given the large amount of materials available on this topic, the present review must necessarily be considered introductory and partial. The discussion aims to provide useful indications on how to distinguish chert from flint and chert from jasper; secondly, the information provided by Pliny and the archaeometric state of the art on these materials is reviewed. The last section examines quartz varieties: hyaline quartz (rock crystal), milky quartz, smoky quartz, rose and pink quartz, amethyst, citrine, prasiolite and blue quartz. An exhaustive mineralogical discussion on quartz is beyond the scope of this review; conversely a review of the historical information is provided, together with a brief list of major supply areas, a summary of the archaeometric studies performed on these materials, as well as an indication of the geological literature which can be used proficiently for provenance studies.