48 Mr. J . Buckman on the Structure and Arrangement 



there worked. It is probable that this drift is the debris of that 

 tertiary rock known in Wiltshire as Sarsen stone, of which the 

 huge stones of Abury Camp constitute the more enduring mo- 

 nument. 



3. This, though differing so much in colour from No. 1, yet 

 seemed so identical in lithological structure as to induce me to 

 try to ascertain from experiment whether or not they were the 

 same, when, on roasting a portion of the rock No. 1 in the lire 

 for a few minutes, it gradually assumed the colour of the gray 

 tesserae, the change no doubt being due to some alteration in the 

 chemical conditions of the iron with which the stone is slightly 

 charged. 



4. The dark colour of the lias entered largely into the compo- 

 sition of these pavements, as much of the outline of the design 

 and the darker bands of the border ornaments are composed of 

 this stone, which, judging from an Ammonite found in one of the 

 tesserae, was obtained from some one of the thin layers of argil- 

 laceous limestone with which the clay-beds of the third division 

 of the lower lias in the vale of Gloucester are separated, and no 

 doubt the stone in question was brought from that locality. 



The artificial tessera found at Cirencester entered for the most 

 part into the construction of the finer and more important parts 

 of the details of the figures and designs ; they consist of — 



Colour. Substance. 



1. Black "J 



2. Light red > Terra cotta or baked clay. 



3. Dark red J 



4. Brilliant ruby-red ... Glass. 



1. This is a much darker shade than that of the lias, and was 

 consequently used in those portions of figures where bold relief 

 was required ; it seems to be composed of a dark-coloured clay, 

 only slightly, if at all burnt ; as the tesserae are very fragile, this 

 would almost lead to the conclusion that these were not arti- 

 ficially coloured but made of a clay containing a large quantity of 

 protoxide of iron, which is black, and they were consequently 

 burnt in smother kilns, or the black would change to red by the 

 protoxide becoming peroxidized. The identity of constitution of 

 these tesserae with black pottery is very apparent. 



2 and 3. The two reds are made from clays containing more 

 or less of iron, and perhaps this substance may have been added 

 in these and in the instance above noticed, where it was desirable 

 to deepen the tint ; of course the red is due to the peroxidation of 

 the iron salts. 



4. In only one medallion of the Cirencester pavements has 

 glass been made to play a part, and that is just when the trans- 



