Analyses of 37 samples of the McCartys basalt collected along the length of the flow indicate the existence of considerable longitudinal variation. Near the vent:, the basalt is a quartz-normative tholeiite containing plagioclase phenocrysts; at distances greater than 4 km from the vent, it is an olivine-normative tholeiite containing olivine phenocrysts. Chemically, the quartz tholeiite is richer in silica, alumina, and lime but poorer in magnesia and ferrous oxide than is the olivine tholeiite. Vertical chemical variations are not significant.

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