This paper has arisen from a need for more exact knowledge of the development of the normal human hip joint than, could be gained from the literature and text-books of embryology. It The formation of the acetabulum, head and neck of the femur, the ligamentum teres, retinacula of Weitbrecht, glenoid labrum, the synovial membrane, and the transverse acetabular ligament are described in detail. The entry of blood vessels into the head and neck of the femur and the suggested predisposition of the head of the femur to dislocation in one embryo are noted in brief.The illustrations of the 6.75 to 45 mm. embryos have been selected from the excellent material available in the Minot Embryological Collection of the Harvard Medical School. Six embryos, 45 mm., 70 mm., 81 mm., 90 mm., 167 mm., and 237 mm. respectively, were cut in serial sections in the Laboratory of Pathology of The Children's Hospital, Boston. The observations on blood vessels and later development have been made from these sections. The crown rump measurement as correlated by Keibel and Mall has been used as a standard. LiteratureWriting on the embryology of the extremities K1lhiker," in 1861, made a statement to the effect that "the ground work of all the bones of the extremities arises at first from one single undifferentiated 'body. When cartilage formation first begins throughout this mass, it will organize itself into several parts so that it will develop into just as many single cartilages or bones." This simple forceful statement is the essence of skeletal embryology.

In some situations, tilting of fault blocks can create "hydrologic windows" through low-permeability confining units (e.g., lacustrine/playa facies) when permeable strata are connected across fault blocks (Figure 1). Along the Rio Grande Rift in New Mexico it has been recognized for some time that fault block motion has served to localize hydrothermal spring activity [Barroll and Reiter, 1990; Witchef, 1988].In this paper we argue that progressive fault block motion controls the present-day and paleohydrology of the Rio Grande Rift by modifying the connections between aquifers and confining units. This hypothesis is illustrated and tested using a basin-scale mathematical model which represents both the past and present-day groundwater flow systems across the Rio Grande Rift near Socorro, New Mexico. A suite of numerical experiments is presented which investigates the hydraulic behavior of fault zones, fault block geometry, and 2641

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We study the influence of vertical fault block motion on the hydrodynamics of continental rift systems in this paper using finite element analysis. Numerical solutions to governing groundwater flow and heat transport equations within independently moving fault blocks are obtained using triangular elements. Hybrid, four-node "slip elements" developed for this study are employed along fault surfaces; standard three-node elements are used everywhere else. The model is used in a sensitivity study to assess the effects of permeability variations, subsidence/erosion rates, and water table configuration on basin hydrodynamics. Idealized simulations of rifting over a 1.5 m.y. period were constructed using two (horst and graben) fault blocks that are vertically offset by 4.5 km during the simulation period. Numerical results indicate that significant interaction between different fluid flow-impelling mechanisms (compaction-, topography-, and density-driven flow) occurs over a permeability range of t0 -16'5 to t0 -14'5 m 2. Many of the flow fields, 145 2 especially those in high-permeability (tO-ß rn ) horst'and graben blocks, produce areas of high and low heat flow relative to conductive conditions. Models which incorporate alternating layers of permeable and less permeable layers indicate that the truncation/reconnection of aquifers can induce transient thermal and hydrologic behavior. The sensitivity results suggest a plausible mechanism for transient fluid flow on geologic timescales (10 s to 107 years) within faulted basins and may help to explain the occurrence of some banded ore deposits or multiple episodes of sediment diagenesis preserved within the sedimentary rock record. Paper number 95WR02114. 0043-1397/95/95WR-02114505.00 Garven, 1992, 1994] methods. Only a handful of studies have represented large-scale displacements along faults; these have focused on hydromechanical processes within foreland basins [Ge and Garven, 1994] and accretionary prisms [Borja and Dreiss, 1989]. In this paper we study the effects of vertical fault block displacements on hydrothermal fluid flow within extensional tectonic settings using finite element analysis. Calculations made in a numerical sensitivity study reveal that largescale displacements of horst and graben blocks may have important consequences for rift basin paleohydrology and transient fluid flow. Normal faults within continental rift basins form as a result of extension of the Earth's crust. Lithosphere extension and mantle necking (and associated high heat flow) weaken crustal rocks and permit isostatic adjustment accommodated along faults. Along the basin-framing faults, displacements can exceed several kilometers and produce large topographic relief between the border fault scarps and the center of the rift. Several kinematic models have been proposed to describe fault block motion and deformation during rifting [Wernicke and Westaway and Kusznir, 1993]. These studies have provided useful criteria for estimating the amounts of lithospheric extension based on fault bl...