Relatively to the super-terranean constructions these excavations appear as prior in time, so that we may regard the enormous spaces laid out above the soil as an imitation and efflorescence of similar tracts of land beneath it. In excavations there is no positive building, but we have rather a given material taken away. And to nest thus in the ground, to excavate is more natural to man than to seek for a material, and with it to construct and inform a mass of buildings. In this respect we may assume the cave to be utilized prior to the hut or dwelling. Caves are the extension of spatial covering instead of a limitation of such, or an extension which grows up as a limit and enclosure, in which the enclosure is already present. The subterranean construction consequently inclines to start with what is already present, and, in so far as it leaves the fundamental material as it finds it, is not erected with the freedom applicable to a configuration raised above the surface of the soil. In our view, however, these constructions already belong to a further stage of the art of building, however much they may also have features of a symbolical type, because they no longer are placed there as independently symbolical, but already possess the aim or purpose of an enclosure, a partition, a roof, within which more symbolical figures as such are set up. That which is connoted under the conception of temple and dwelling, both in the Greek and more modern use of the terms, we have here in their most natural form. We may include in the above class the caves of Mithras, although we find them in a very different locality. The worship and ritual of Mithras originates in Persia. A cultus, however, of a similar kind was also promulgated in the Roman Empire. In the Paris Museum we find a very famous bas-relief, which represents a youth in the act of striking an ox with a steel weapon. It was discovered in the Roman Capitol in a deep grotto beneath the temple of Jupiter. In these Mithras caves vaults are also met with, and passages which on the one hand appear definitely to symbolize by suggestion the course of the stars, and from another point of view also (precisely as still in our own time takes place in our free-mason lodges, where people are conducted through many passages and have to see dramatic scenes and much else) the ways, which the soul must pass through in its purification, albeit it may be true enough that this fundamental meaning is more fully and directly expressed in sculpture and other work than in architecture simply. In a connection somewhat similar we may also mention the Roman catacombs, the fundamental idea of whose construction was certainly something quite other than that of being subservient to aqueducts, sepulchres or any system of drainage.