198 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



It is however certain that after its deposit, the temple must have 

 been filled up to the depth of from 5 to 9 feet, a fact which is plainly- 

 indicated by the form of the lower edge of the great calcareous 

 deposit. 



Now the lowest stratum in the section adjacent to the temple, 

 marked No. 6 in Plate II. and described at par. 41, has every ap- 

 pearance of a volcanic tuff: although I did not measure its height, 

 I remember it was large compared to most of the others. It pro- 

 bably arose from an eruption of the Solfatara, and falling uniformly 

 over the whole area of the temple, would leave a considerable eleva- 

 tion in the central part, the floor of which was already elevated S-J 

 feet above the rest of the temple. 



If this central part contained, as some have imagined, a circular 

 temple with a marble roof, the weight of 5 or 6 feet of tuff might 

 have broken it down, and thus have caused a still greater elevation 

 in the centre. 



65. The next fact given by the observations is the great incrus- 

 tation. It is much harder and thicker than the preceding, and occurs 

 (see par. 38) behind D 4 — in the chamber C 8 — in C C, to the west 

 of the great entrance' — and also on the standing columns, and on the 

 fragments described in par. 8, 10, 15, 16, 18,23, 25, 26, 27. 



This incrustation cannot have arisen from sea-water, for the reason 

 stated in par. 58. It may have arisen from the hot spring, and this is 

 rendered highly probable from the following fact. 



In that singular building called the Piscina mirabile, which is at 

 a considerable distance from the sea, and which is supposed to have 

 been used by the Romans as a reservoir for fresh water, there occurs 

 an incrustation nearly similar in external characters. 



Mr. Faraday in speaking of it says, " This is a chemical composi- 

 tion as like the last (that of the temple of Serapis) as possible ; I do 

 not find a word to alter. The state of aggregation is different and 

 the successive deposits are not so evident : it is also more cry- 

 stalline. 



"Your first question, whether the first and second deposits (those 

 of Serapis and the Piscina mirabile) are the same substances nearly 

 in the same proportion, is already answered in the affirmative. Your 

 second, of ' whether the combinations they contain are compatible 

 with sea-water, or could they have been deposited in it?' requires a 

 little more reservation : I cannot say that the carbonate of lime is 

 incompatible with sea-water, or that it could not have been deposited 

 from it. But I never heard of such a deposit from sea-water, nor 

 can we now-adays either naturally, or during the evaporation which 

 goes on in salt-works, &c. &c. On the other hand, they represent 

 perfectly such deposits as are taking place continually from waters 

 holding carbonate of lime in solution by carbonic acid, and I cannot 

 help thinking that such has been their source. In giving this opinion 

 I am guided merely by the appearances of the deposits and their 

 chemical characters, for I know nothing of the circumstances under 

 which they occur, although twenty years ago I happened for a few 

 hours to be at the temple of Serapis." 



