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CORRESPONDENCE Pros and cons of 24/7 working stir up debate p.280 CULTURE Martin Kemp muses on 15 years of artists in lab schemes p.278 MUSIC In conversation with climate-change composer Paul D. Miller p.279 HISTORY Copernicus biography from Dava Sobel mixes fact and fiction p.276 Natural gas from shale is widely promoted as clean compared with oil and coal, a 'win-win' fuel that can lessen emissions while still supplying abundant fossil energy over coming decades until a switch to renewable energy sources is made. But shale gas isn't clean, and shouldn't be used as a bridge fuel.Shale rock formations can contain vast amounts of natural gas (which is mostly methane). Until quite recently, most of A fter a career in geological research on one of the world's largest gas supplies, I am a born-again 'cornucopian' . I believe that there is enough domestic gas to meet our needs for the foreseeable future thanks to technological advances in hydraulic fracturing. According to IHS, a business-information company in Douglas County, Colorado, the estimated recoverable gas from US shale source rocks using fracking is about 42 trillion cubic metres, almost Should fracking stop?Extracting gas from shale increases the availability of this resource, but the health and environmental risks may be too high. POINT Yes, it's too high riskNatural gas extracted from shale comes at too great a cost to the environment, say Robert W. Howarth and Anthony Ingraffea. COUNTERPOINT No, it's too valuableFracking is crucial to global economic stability; the economic benefits outweigh the environmental risks, says Terry Engelder.

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A B S T R A C TThe marine Middle and Upper Devonian section of the Appalachian Basin includes several black shale units that carry two regional joint sets (J 1 and J 2 sets) as observed in outcrop, core, and borehole images. These joints formed close to or at peak burial depth as natural hydraulic fractures induced by abnormal fluid pressures generated during thermal maturation of organic matter. When present together, earlier J 1 joints are crosscut by later J 2 joints. In outcrops of black shale on the foreland (northwest) side of the Appalachian Basin, the eastnortheast-trending J 1 set is more closely spaced than the northwest-striking J 2 set. However, J 2 joints are far more pervasive throughout the exposed Devonian marine clastic section on both sides of the basin. By geological coincidence, the J 1 set is nearly parallel the maximum compressive normal stress of the contemporary tectonic stress field (S Hmax ). Because the contemporary tectonic stress field favors the propagation of hydraulic fracture completions to the east-northeast, fracture stimulation from vertical wells intersects and drains J 2 joints. Horizontal drilling and subsequent stimulation benefit from both joint sets. By drilling in the north-northwestsouth-southeast directions, horizontal wells cross and drain J 1 joints, whenever present. Then, staged hydraulic fracture stimulations, if necessary, run east-northeast (i.e., parallel to the J 1 set) under the influence of the contemporary tectonic stress field thereby crosscutting and draining J 2 joints.

To compare the orientation and development of jointing with the orientation and magnitude of finite strain recorded in the Upper Devonian rocks of the Appalachian plateau, New York, we mapped systematic joint sets on an area of 20,000 km2. In this area, Wedel [1932] mapped folds with limb dips of less than a degree and axes that change strike by 30° from 090° in the east to 060° in the west. We observed two different cross‐strike joint sets that maintain their approximate cross‐strike position from east to west. Yet, in detail the angle between the sets is 18°±2° in the east and 30°±4° in the west. In many outcrops one joint set parallels the direction of maximum compressive strain (εƒ) as recorded by deformed fossils, whereas the other joint set never parallels εƒ. Rare calcite‐filled joints are oriented parallel to the direction of εƒ. In addition, the calcite‐filled joints both cut and are cut by solution cleavage. These observations suggest that the joint set paralleling εƒ formed during the deformation event represented by the deformed fossils. The joint set that does not parallel εƒ somehow reflects a deformational event other than that producing fossil distortion, as suggested by strain relaxation experiments. In an outcrop of the Machias formation, where the direction of εƒ is 15° from the trace of a cross‐strike joint set, the tensor average from 14 subsurface strain relaxation tests shows in situ maximum comprehensive strain (εoc = 10−4) parallel to cross‐strike joints but not the direction of εƒ. Strain relaxation is coaxial with a fabric anisotropy indicated by sonic velocity tests. Our idea is that the orientations of cross‐strike joints parallel to εoc were controlled by the same rock property that causes strain relaxation on overcoring, whereas the orientations of the cross‐strike joints parallel to εƒ were controlled by the stress field causing the fossil distortion. Placing these structural data in a regional context allowed us to construct a dynamic and kinematic model for the structural evolution of the New York plateau. The model indicates variation in both boundry conditions and material behavior through a series of four distinct deformational events, which begin prior to lithification and end in the Recent. Thus our analysis suggests that the structural features we have used represent a set of highly sensitive tools for investigating the deformational history of the Appalachian foreland.