270 TRAVELS THROUGH 



on calcareous-hills; fuch as monte Morello, and 

 others. I have noticed already in one of my for- 

 mer Letters that macigno is a fpecies of flate, com- 

 pofed of an argillaceous earth, much mica, and 

 fome lime j wherefore it is flowly and gently 

 affected by aqua fortis. Near Fiefole the macigno 

 is in , the uppermoft ftrata (hivery or thinly-lamcl- 

 lated ; that is to fay, it confifts of fmall beds, 

 alternating with argillaceous ftrata, which are 

 called Bardelloni. In greater depth, it grows found 

 and compact without any vifible leparadon of 

 beds, fo that it can be cut into large mafTes and 

 columns, which they do by many iron wedges 

 driven into it on the fame line. Now and then 

 occur in this found macigno argillaceous nodules, 

 and very frequently little black fpots and thin beds 

 or veins of coals or clay faturated and black-co- 

 loured petroleum. The macigno appears in two 

 different colours. I. Pietra Bigia is grey-yellow, 

 on account cf fome admixed iron-ocher; com- 

 monly found uppermoft, but Ibmetimes clofe flick- 

 ing with the following variety. 2. Pietra Serena^ 

 petra columlina, or tur china, is blueifli grey ; 

 commonly ufed in the buildings at Florence, but 

 moulders away in open air, in which it contracts 

 a black colour. Many columns made of it are to 

 be feen in the church di S. Spirito. The petra 

 bigia is, on account of its ochraceous mixture, 



harder 



