1818.] ON THE FORMATION OP ROCKS, ^ 



34. Sandstone of transition^ an aggregate of small 

 jiarticles rounded by attrition^ united generally by a 

 siliceous cement, alternating with clay- slate and gray- 

 wacke sliist. This rock has been found to contain th^ 

 remains of organic matter^ and must therefore be consi- 

 dered of Neptunian origin. 



This is rather a partial foruiation^ found generally 

 in thin strata, alternating with the transition shist^ 

 though it forms in some places west of the Allegliany 

 mountains, in North America, considerable ranges of 

 small hills, and constitutes a great proportion of the 

 rolled pebbles found in rivers w hich ruii over the tran- 

 sition formation. 



25. Limestone of transition, resembling a little the 

 limestone by precipitation of the first order, though 

 not so similar as that of the secondary: and containing, 

 though in small quantities, shells and the remains of 

 other organic matter, which shew it to be of Neptunian 

 origin. 



I This limestone is mixed with the giaywacke, and 

 clay slates, in almost every proportion, from the thin- 

 nest sliistoze stratification, to the most powerful and so- 

 lid beds; forming immense blocks free from cracks or 

 fissures ; and it is probable, that the small grained stat- 

 uary marble both of Italy and Greece, belongs to this 

 formation. When this formation touclies the compact 

 secondary limestone, ^vithout any intervention of gray- 

 wacke sliist or slaty rocks, the passage is gradual and 

 almost imperceptible, leaving much doubt and difficul- 

 ty about the place wliere the line of separation ought 

 to be drawn. It is of all the members of t]ie trausitiou 



Yol.l. Y 



