the mantle of the earth is mainly made of silicate & the crust of the earth is made of basalt or granite ->  the mantle of the earth is mainly made of silicate and the crust of the earth is made of basalt or ganite  basalt is a kind of material & granite is a kind of material & silicate is a kind of material ->  silicate, basalt, and granite are kinds of materials  the mantle of the earth is mainly made of silicate and the crust of the earth is made of basalt or ganite & silicate, basalt, and granite are kinds of materials & different media are made of different materials ->  the mantle and the crust are different media  the mantle and the crust are different media & waves travel at different speed in different medium ->  waves will travel at different speed in the mantle and the crust  waves will travel at different speed in the mantle and the crust & an earthquake wave is a kind of wave -> earthquake waves will travel at different speed in the mantle and the crust