“In my opinion, the date when copper or bronze was first known in Britain is a very remote one, as no country in the world presented greater facilities for their discovery. The beginning of their application to practical uses should, I think, be placed at least as far back as 1800 B.C., and that date I am inclined to give, until further evidence is forthcoming, as the approximate date of the erection of Stonehenge.” Now the date arrived at by Mr. Penrose and myself on astronomical grounds was about 1700 B.C. It is not a little remarkable that independent astronomical and archæological inquiries conducted in the same year should have come so nearly to the same conclusion. If a general agreement be arrived at regarding it, we have a firm basis for the study of other similar ancient monuments in this country. I have previously in this book referred to the fact that the trilithons of the naos and the stones of the outer circle are all built up of so-called “sarsen” stones. To describe their geological character, I cannot do better than quote, from Mr. Cunnington’s “Geology of Stonehenge,”[16] their origin according to Prestwich. “Among the _Lower Tertiaries_ (the Eocene of Sir Charles Lyell) are certain sands and mottled clays, named by Mr. Prestwich the Woolwich and Reading beds, from their being largely developed at these places, and from these he proves the sarsens to have been derived; although they are seldom found _in situ_, owing to the destruction of the stratum to which they belonged. They are large _masses of sand concreted together_ by a siliceous cement, and when the looser portions of the stratum were washed away, the blocks of sandy rocks were left scattered over the surface of the ground. “At Standen, near Hungerford, large masses of sarsen are found, consisting almost _entirely_ of flints, formed into conglomerate with the sand. Flints are also common in some of the large stones forming the ancient temple of Avebury. “The abundance of these remains, especially in some of the valleys of North Wilts, is very remarkable. Few persons who have not seen them can form an adequate idea of the extraordinary scene presented to the eye of the spectator, who standing on the brow of one of the hills near Clatford, sees stretching for miles before him, countless numbers of these enormous stones, occupying the middle of the valley, and winding like a mighty stream towards the south.” These stones, then, may be regarded as closely associated with the local geology. The exact nature of the stones, called “blue stones,” can best be gathered from a valuable “Note” by Prof. Judd which accompanies Prof. Gowland’s paper. These blue stones are entirely unconnected with the local geology; they must, therefore, represent boulders of the Glacial drift, or they must have been brought by man, from distant localities. Prof. Judd inclines to the first opinion. The distinction between these two kinds of stone are well shown by Prof. Gowland:--