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.

4018. 'In front of the flocks; and they came on heat as they came to drink' means even to the point of an intense desire in the affection for truth that a joining [to the goods and truths within the natural] might be effected. This is clear from the meaning of 'coming on heat as they came to drink' as an intense desire. The meaning of 'being on heat' as an intense desire is self-evident; and for the meaning of 'coming to drink' as the affection for truth, see immediately above in 4017. The reason why 'in front of the flocks' means that a joining to the truths and goods within the natural might be effected is that this phrase implies seeing and consequent arousal of affection, for this is the manner in which spiritual things are joined to a person. What is more, every implantation of truth or good in a person, as well as every joining of them to him, is effected by means of affection. The truths and goods which a person has learned but for which he has no affection do indeed enter the memory, but they are lodged there as insecurely as a feather on top of a wall which is blown off by the slightest puff of wind.

[2] As regards the things that enter the memory the position is this: Those for which there is no affection pass into the unlit parts of the memory when they enter it, whereas those for which there is affection pass into the light there. Things present in that light are seen and appear clearly and distinctly when any matter of a similar nature is brought up, but not so the things lying around in the unlit parts. Such is the effect that affection belonging to love has. From this it may be seen that all implanting of truth and joining of it to good is effected through affection; and the greater that affection, the stronger the tie joining the two together.

[3] The intense desire of the affection is in this case inmost affection. But truths are not capable of being implanted and joined to good except by means of affections for truth and good, which affections well up from charity towards the neighbour and love to the Lord as their sources. But evils and falsities are implanted by means of affections for evil and falsity, and these affections well up from self-love and love of the world as their sources. This being so, and the subject at this point in the internal sense being the joining of good and truth within the natural man, mention is therefore made here and in what follows of the flock being on heat when they came to drink, by which such considerations are meant.