A passage in Humboldt, which I met with long after, seems to clear up the mystery.

Speaking of the great teocalli of the city of Mexico, he says, quoting an old description, that the Moon had a little temple in the great courtyard, which was built of shells.
Those that we found may be the remains of a similar structure on the top of the pyramid.
Prickly pears, aloes, and mesquite bushes have overgrown the pyramids in all directions, as though they had been mere natural hills.

In Sicily one may see the lava fields of Etna planted with prickly pears: in the ordinary course of things, it requires several centuries before even the surface of this hard lava will disintegrate into soil; but the roots of the cactus soon crack it, and a few years suffice to break it up to a sufficient depth to allow of vineyards being planted upon it.
Here the same plant has in the same way affected the porous amygdaloid with which the pyramids are faced, and has cut up the surface sadly; but the vegetation which covers them will at any rate defend them from the rains, and now centuries will make but little change in the appearance of these remarkable buildings.
Near Nice there is a hill which gives a wonderfully correct idea of the appearance of the terraced teocallis of Mexico, as they must have looked before time effaced the sharpness of their lines.

Where the valley of the Paglione and that of St.Andre meet, the hill between them terminates in a half pyramid, the angle of which lies toward the south; and the inhabitants--as their custom is in southern Europe, have turned the two slopes to account, by building them up into terraces, to prevent the soil they have laboriously carried up from being swept down by the first heavy rain.

Seen from the proper point of view the resemblance is complete.
From the south side of the Temple of the Moon runs an avenue of burial-mounds, the Micaotli, "the path of the dead." On these mounds, and round the foot of the pyramids themselves, the whole population of the once great city of Teotihuacan and its neighbourhood used to congregate, to see the priests and the victims march round the terraces and up the stairs in full view of them all.