We crossed the river and passed through the mountains beyond, and finally we came out upon a great level plain which stretched away as far as the eye could reach.  I cannot tell you in what direction it stretched even if you would care to know, for all the while that I was within Pellucidar I never discovered any but local methods of indicating direction—­there is no north, no south, no east, no west.  Up is about the only direction which is well defined, and that, of course, is down to you of the outer crust.  Since the sun neither rises nor sets there is no method of indicating direction beyond visible objects such as high mountains, forests, lakes, and seas.

The plain which lies beyond the white cliffs which flank the Darel Az upon the shore nearest the Mountains of the Clouds is about as near to any direction as any Pellucidarian can come.  If you happen not to have heard of the Darel Az, or the white cliffs, or the Mountains of the Clouds you feel that there is something lacking, and long for the good old understandable northeast and southwest of the outer world.

We had barely entered the great plain when we discovered two enormous animals approaching us from a great distance.  So far were they that we could not distinguish what manner of beasts they might be, but as they came closer, I saw that they were enormous quadrupeds, eighty or a hundred feet long, with tiny heads perched at the top of very long necks.  Their heads must have been quite forty feet from the ground.  The beasts moved very slowly—­that is their action was slow—­but their strides covered such a great distance that in reality they traveled considerably faster than a man walks.

As they drew still nearer we discovered that upon the back of each sat a human being.  Then Dian knew what they were, though she never before had seen one.

“They are lidis from the land of the Thorians,” she cried.  “Thoria lies at the outer verge of the Land of Awful Shadow.  The Thorians alone of all the races of Pellucidar ride the lidi, for nowhere else than beside the dark country are they found.”

“What is the Land of Awful Shadow?” I asked.

“It is the land which lies beneath the Dead World,” replied Dian; “the Dead World which hangs forever between the sun and Pellucidar above the Land of Awful Shadow.  It is the Dead World which makes the great shadow upon this portion of Pellucidar.”

I did not fully understand what she meant, nor am I sure that I do yet, for I have never been to that part of Pellucidar from which the Dead World is visible; but Perry says that it is the moon of Pellucidar—­a tiny planet within a planet—­and that it revolves around the earth’s axis coincidently with the earth, and thus is always above the same spot within Pellucidar.

I remember that Perry was very much excited when I told him about this Dead World, for he seemed to think that it explained the hitherto inexplicable phenomena of nutation and the precession of the equinoxes.