From the investigation of 20 samples of garnets from the Shinyama ore deposits of the Kamaishi mine by means of the decrepitation method, the writer concludes as follows.
(1) The logarithmic expression of the decrepitation frequency shows a linear relation to the temperature at the neibourhood of the knick point of the decrepigraph. The decrepitation temperature of garnet, which is the lowest temperature on the straight line, does not relate to the distance from the igneous body.
(2) There are two types of the decrepitation patterns. The chemical compositions of both types are nearly the same (And₅₀ Gr₅₀-And₃₀ Gr₇₀, determined by X-ray powder patterns). The type I garnet does not decrepitate frequently (less than 15 counts/sec) and is found in the barren skarn zone. On the other hand, the type II decrepitates frequently (more than 30 counts/sec) and is found in the ore bodies.