Abstract
Summary Potassium-argon age determinations of two intrusions of olivine-dolerite, cutting Borrowdale Volcanic rocks near Melmerby, Cumberland, gave a mean age of 296 ± 8 million years, placing them at the Westphalian-Stephanian boundary, and showing their emplacement to be contemporary with that of the Whin Sill, which crops out on the nearby Pennine escarpment. Chemical analyses of these rocks show their similarity to other olivine-dolerites, and variation-diagrams suggest a genetic line of descent from the oversaturated Whin magma. The intrusions may represent part of the western margin of the Whin Sill, particularly as at the time of the intrusion of the sill, Carboniferous and Lower Palaeozoic rocks were probably here juxtaposed along the Deep Slack Fault by Hercynian faulting throwing down to the east.
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