R.H. JAHNS (California Institute of Technology, Pasadena, Calif.): Mr. Moderator, Members and Guests of the Society: The five points of view that were expressed this morning concerning the origin of granite are perhaps most striking for their similarity, at least insofar as the introductory remarks were concerned. There seems to be general agreement that some granite was formed by crystallization from magma, some by replacement of pre-existing crustal material, and perhaps some by other processes. The agreement is less pronounced, unfortunately, on the definitions of such fundamental terms as “granite,” “gneissic granite,” “granite gneiss,” and “gneiss.” Although opinions differ little concerning the composition of true granite, questions of texture are by no means so simple. To what extent can metamorphic textures be present in a rock defined as “granite”? To what extent, even, can such textures be identified with assurance? It is clear that geologists are not yet agreed on single answers to these questions.

Even the term “magma” gives trouble. Evidently there are those who feel that a true magma should be fluid, or almost wholly so, whereas others would extend the term to include most mixtures of solid and fluid material. A few extremists would take a broad view of this term and would then define granite as any silicic rock formed by crystallization from a magma!

For purposes of this discussion, let us take from the statements of this morning an “average definition”, however approximate that might be, for each of these basic terms, and go on . . .