Some twenty or thirty of these are known still to exist in a state of greater or less preservation, but, with one exception, all cut in the rock. In so far as the interior is concerned this is of little or no consequence, but it prevents our being able to judge of their external form or effect,[133] and, what is perhaps worse, it hides from us entirely the mode in which their roofs were constructed. We know that they were formed with semicircular ribs of timber, and it is also nearly certain that on these ribs planks in two or three thicknesses were laid, but we cannot even guess what covered the planks externally. It could hardly have been metal, or any kind of felt, and one is unwilling to believe that they were thatched with grass, though I confess, as the evidence at present stands, this seems to me the most probable suggestion.[134] [Illustration: 41. Plan of Chaitya Hall, Sanchi. Scale 30 ft. to 1 in.] The only structural one is at Sanchi, and is shown in plan in the accompanying woodcut (No. 41). It does not however, suffice to show us how the roofs of the aisles were supported externally. What it does show, which the caves do not, is that when the aisle which surrounded the apse could be lighted from the exterior, the apse was carried up solid. In all the caves the pillars surrounding the dagoba are different from and plainer than those of the nave. They are, in fact, kept as subdued as possible, as if it was thought they had no business there, but were necessary to admit light into the circumambient aisle of the apse. * * * * * As almost all our information regarding these chaityas, as well as the viharas, which form the next group to be described, is derived from the rock-cut examples in Western India, it would be convenient, if it were possible, to present something like a statistical account of the number and distribution of the groups of caves found there. The descriptions hitherto published do not, however, as yet admit of this. I have myself visited and described all the most important of them;[135] and in an interesting paper, communicated to the Bombay branch of the Asiatic Society by the Rev. Dr. Wilson, he enumerated thirty-seven different groups of caves, more or less known to Europeans.[136] This number is exclusive of those in Bengal and Madras, and new ones are daily being discovered; we may therefore fairly assume that certainly more than forty, and probably nearly fifty, groups of caves exist in India Proper.