The first altar mentioned in the Word was the one built by Noah after he came out of the ark, after being saved from the great flood. On that altar, he sacrificed clean animals to the Lord.

Mountains represent the Lord because of their height; we need to raise our thoughts above worldly things when "talking" with the Lord. An altar is a small artificial mountain. When it's used in worship, it can call to mind this raising of thought. The fire and smoke that rise from an altar are symbolically being sent to the Lord.

Most altars were made from unhewn stones. Stones represent truths. Unhewn stones - ones that have not been shaped by men - represent truths from the Word, truths that have not been adulterated.

The clean beasts to be sacrificed represent good things, charitable acts done because they are right. The clean birds represent thoughts about doctrine and actions, and about what is right. Presenting these things is an acknowledgment that we have them from the Lord, and a giving thanks to Him for them.

In the Israelitish Tabernacle, the altar of burnt offering represented the acknowledgment of good and the altar of incense that of truth. For this reason this larger altar, which was outside by the door, was made of brass which signifies natural good, while the altar of incense was made of gold, which signifies love to the Lord from whom comes truth.

249. 'Eating dust all the days of its life' means that the sensory part had become such as could live from nothing else than the bodily and the earthly, and so had become hellish. This too is clear from the meaning of 'dust' in the Word, as in Micah, 7:14, 16-17.

Shepherd Your people as in the days of eternity. The nations will see and be ashamed at all their might, they will lick the dust like serpents of the earth they will be shifted from their strongholds.

'Days of eternity' stands for the Most Ancient Church, 'nations' for people who put their trust in the proprium, who are referred to as 'licking dust like a serpent'. In David,

Barbarians will bow down before God, and His enemies lick the dust. Psalms 72:9.

'Barbarians' and 'enemies' stand for those who look solely to earthly and worldly things. In Isaiah,

Serpents, dust will be their bread. Isaiah 65:25.

Because 'dust' meant people who did not look to spiritual and celestial things but to bodily and earthly, the Lord commanded the disciples to shake the dust off their feet if a city or house was not worthy, Matthew 10:14. For more on 'dust' meaning that which is condemned and hellish, see at verse 19.