The sill shows the normal type of spatial relationship found in composite intrusions of the British Tertiary, i.e. basic-acid-basic. The basic rocks are Talaidh type tholeiites while the acid rock is a rhyolite. Xenoliths appear to be concentrated in the upper and lower tholeiites. This fact has suggested to the authors of the Mull Memoir that a process of gravitational differentiation resulted in the formation of the basic and acid magmas. The evidence is inconclusive as to what process operated in magma genesis. The mineralogy of the xenoliths found in the sill is described in detail.
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