Abstract
According to the quantum theory of solid, the ratio of bulk modulus of a crystal to its rigidity is approximately proportional to density. By computing the ratio from the observed velocity values in the earth's mantle, several discontinuities can be found.
If the density does not decrease in the mantle according to depth, the decrease of the ratio cannot be explained without assuming some change in composition or succesive occurrences of polymorphic transitions. The ratio decreasses from 500km to 800km in deph. It seems reasonable to assume that the layer from 500km to 800km is a transitional one from the viewpoint of crystal theory. No other discontinuous decrease of the ratio can be seen except this layer. 'There are two discontinuities at 1, 000km and 1, 800km. It seems probable that the mantle has one transitional layer and two transitional points and so five layers in all. These layers are
a) 33km-500km
b) 500km-800km
c) 800km-1000km
d) 1000km-1800km
e) 1800km-2800km
