The right branch of the chart is the continuous series of crystallization because the plagioclase minerals maintain the same basic crystal structure but change continuously in calcium and sodium content away from calcium-rich plagioclase towards a sodium-rich variety.
The left branch of the chart is the discontinuous series of crystallization because the reactions result in minerals of distinctly different structure.
Potassium feldspar, muscovite mica, and quartz do not react with the melt. By the time they crystallize, there is little liquid left.
Bowen's Reaction Series allows geologists to recognize why mineral variations exist in igneous rocks. A volcanic rock from an early eruption may be rich in iron, magnesium, and calcium and thus produce basalt. Therefore, later eruptions might be depleted in iron, magnesium, and calcium but enriched in potassium, sodium, and silica. These rocks would be less basaltic and more andesitic in composition.
Source: The Earth Through Time by Harold Levin
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