The study of minerals, using X-ray goniomcters in place of optical I instruments, has proceeded far in recent years. It is now a common practice among mineralogists and petrologists to confirm the nature of a given specimen by X-ray methods when there is some doubt about the identification by the older methods. The occurrence of pseudomorphs sometimes leads to misidentifications even in crystals of large size. If the material is cryptoerystalline it may be almost impossible to make an identification by optical methods. In such cases X-ray photographs may be of great assistance.