Chief Seattle, in his speech of 1854, looks back at the golden past of his tribe, and at the same time, mourns the gradual extinction of the Red Indians by repeatedly mentioning how his race filled the country once upon a time. To express the past golden days of the tribal people, Seattle has used several similes where he compares and contrasts the state of the native people in the past with that at present.

There was a time when our people covered the land as the waves of a wind-ruffled sea cover its shell-paved floor, but that time long since passed away with the greatness of tribes that are now but a mournful memory.

The speaker here compares the past situation of the tribals when they were in their glorious phase to the waves of a wind-ruffled sea. He means to express how lively and energetic their people were then.
Again, Seattle says:

“Our people are ebbing away like a rapidly receding tide that will never return.”

Here he compares the present situation of his tribe, at the time of the speech, to a rapidly receding tide, that is going back from the shore, losing its might and with no hope to return again. This symbolizes the decay of the native people.