202 ON THE FORMATION OF ROCKS. [July, 



formation^ the one wliicli most resembles in structure 

 those of tlie secondarj. 



26. Gypsum of transition^ resembling in compo- 

 nent parts^ though differing a little in structure from, 

 the gypsum of alluvial and secondary : as it alternates 

 and is mixed with clay slate, in Avhich the remains of 

 organic matter has been found, this connects it with the 

 rocks of Neptunian origin. 



This is a considerable formation, generally found 

 in mountainous countries ; and from the facility with 

 which it is dissolved by water, is in a broken and con- 

 fused state, often out of its original place, which is 

 perhaps the reason it has been so often supposed to be 

 inclosed in primitive rocks, which tlie result of all my 

 observations incline me to doubt. 



It is probable that all the gypsum in Tuscany be- 

 longs to this formation ; the powerful bed on the top 

 cf mount Cenis, I should think also of the same spe- 

 cies. This gypsum having on each side a blue lime- 

 stone, with dark coloured shist alternating with calc- 

 spar, it seems to be connected with the transition rocks, 

 as w ell as the many powerful beds which are found in 

 the valley of Lanz, from Lanz le Bourg to JHgue- 

 belle. 



The gypsum found in the valley between St. Mar- 

 tins and Sion, in the Switz Alps, is surrounded by 

 what 1 consider transition rocks ; and that perhaps in 

 the valley of Cliamouny, and in the pass between Air- 

 olo and Desentis, from the nature of surrounding rocks , 

 must be classed in the gypsum of transition. As these 

 three last mentioned localities are in a line, running 

 nearly with the stcatijQication of the chain of mountains.^ 



