J818.3 ON THE FORMAlTION OF ROCKS, 28^ 



Essay on the Formation of Rocks, or an Liquiry into 

 theprohahle Origin of their present Form anA^truc-^ 

 ture. By William Maclure, Continued, 



Nature composes, forms, or aggregates those rocks ei** 

 tlier by mechanical deposition, as in sand, gravel,or play, 

 Nos. 1, 2 & 3; by precipitation from a chemical solution, 

 as in Nos. 9, 10 & 11; or by a mixture of both modes of 

 aggregation, as in Nos. 4, 5, 6, 7? and 8, X^i^^se aggre- 

 gations of sandstone, puddingstone &c, are more oyless 

 hard and adhesive, according to the nature of the pre- 

 cipitate which unites them; and nearly resemble the ag- 

 gregates in the Sd order: but the depositions of impal- 

 pable powder, such as Clays &c, generally remain in a, 

 soft state, having less resemblance to the slaty and ar- 

 gillaceous rocks of the other orders of Neptunian origin. 

 The calcareous precipitations are not generally so hard 

 and adhesive as those of the second oyder, though they 

 have much the same texture and extern^-l appearance; 

 but the siliceous precipitations in petrified wood, and 

 other forms, are equally hard and con^pact, having a di- 

 rect analogy witJi the siliceous rocks of the second or- 

 der; they are likewise the only species of rocks, positive- 

 ly known to be of Neptunian origin, which rese^ible a 

 little in ttjxture, hardness and external appearance, n^a- 

 ny of those of volcanic origin. 



Rocks of the following description, may be consider- 

 ed of Neptunian origin by ppsitive and ratio^^l analo- 

 gy, viz. 



Those containing shells and other animals, known 

 only to exist in vrater, or the remains of other organic 

 matter, destructible by lire. 



Aggregates of sand or gravel, rounded by attrition^ 



Vol. L V 



