Library Eusebius of Caesarea Life of Constantine
Book 3, Chapter 36: Description of the Walls, Roof, Decoration, and Gilding of the Body of the Church.
For at the side opposite to the cave, which was the eastern side, the church itself was erected; a noble work rising to a vast height, and of great extent both in length and breadth. The interior of this structure was floored with marble slabs of various colors; while the external surface of the walls, which shone with polished stones exactly fitted together, exhibited a degree of splendor in no respect inferior to that of marble. With regard to the roof, it was covered on the outside with lead, as a protection against the rains of winter. But the inner part of the roof, which was finished with sculptured panel work, extended in a series of connected compartments, like a vast sea, over the whole church; and, being overlaid throughout with the purest gold, caused the entire building to glitter as it were with rays of light.
English translation by Ernest Cushing Richardson, in Nicene and Post-Nicene Fathers, Series II, Vol. I (eds. Philip Schaff and Henry Wace; Buffalo: Christian Literature Publishing Co., 1890). Public domain. Digital text via New Advent Church Fathers.