large enough for the pressure of the magma to force the overlying strata to dome upwards. Gilbert also determined that larger laccoliths formed at greater depth. Both laccoliths and sills are classified as concordant intrusions, since the bulk of the intrusion does not cut across host rock strata, but intrudes between strata. More recent study of laccoliths has confirmed Gilbert's basic conclusions, while refining the details. Both sills and laccoliths have blunt rather than wedgelike edges, and sills of the Henry Mountains are typically up to thick while laccoliths are up to thick. The periphery of a laccolith may be smooth, but it may also have fingerlike projections consistent with Rayleigh-Taylor instability of the magma pushing along the strata. An example of a fingered laccolith is the Shonkin Sag laccolith in Montana, US. The critical radius for the sill to laccolith transition is now thought to be affected the viscosity of the magma (being greater for less viscous magma) as well as the strength of the host rock. A modern formula for the shape of a laccolith is: where is the height of the laccolith roof, is the acceleration of gravity, is the elastic modulus of the host rock, is the horizontal distance from the center of the laccolith, and is the outer radius of the laccolith. Because of their greater thickness, which slows the cooling rate, the rock of laccoliths is usually coarser-grained than the rock of sills. The growth of laccoliths can take as little as a few months when associated with a single magma injection event, or up to hundreds or thousands of years by multiple magmatic pulses stacking sills on top of each other and deforming the host rock incrementally. Over time, erosion can form small hills and even mountains around a central peak since the intrusive rock is usually more resistant to weathering than the host rock. Because the emplacement of the laccolith domes up the overlying beds, local topographic relief is increased and erosion is accelerated, so that the overlying beds are eroded away to expose the intrusive cores. Etymology The term was first applied as laccolite by Gilbert after his study of intrusions of diorite in the Henry Mountains of Utah in about 1875. The word laccolith derived in 1875—1880, from Greek, lákko(s), meaning "pond", plus -lith, meaning "stone". Where laccoliths form Laccoliths tend to form at relatively shallow depths and in some