This paper is a summary of studies made on extensive samples of the main bulk of the Cascan formation of the Cascade Mountains of Oregon. The rocks show many mineral combinations, yet all minerals show that they were once a part of a common magma, altered by incorporated rocks, that was moving toward a fixed mineral objective and stopped in this march at various places. Hence, the various mineral assemblages are only one frame in a cinema-like picture, the final scene of which would have shown all the rocks to be composed of pigeonite and labradorite. The effects of magma contamination, resorption, reaction, and arrested stages are found to be misleading when isolated flows or areas are examined. However, detailed optical studies on many hundreds of selections from many localities resolve the varied and complex results into a simple, straightforward change from a dacitic magma toward a basic andesite end product.

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