“Pressure-shadows” is the name sometimes applied to the fringes or halos differing from the groundmass that often accompany por-phyroblasts in schistose rocks. Excellent examples of pressure-shadows were found with the pyrite porphyroblasts in the Calaveras formation and associated rocks at Carson Hill in the Mother Lode region of California. The pressure-shadows are always widest in the direction of “Streckung1 and are sometimes notably elongated. In the larger shadows it is easily seen with the unaided eye that they are filled with fibrous quartz. This occurrence seemed to offer an excellent opportunity for the study of the orientation of the quartz and of the factors affecting it. To this end the universal stage was used. In the following the material will be described, the method of study outlined, and the results of the measurements presented in graphical form.

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