1818.] ON THE FORMATION OiF ROCKS. 52/ 



Essay on the Formation of Rocks^ or an Inquiry 

 into the probable Origin of their present Form and 

 Structure. By Williara Maclnre. 



(Concluded.) 



Scoria is a kind of vitieous scum tlh^it iloats on tlie 

 surface of all lavas, and is often ejected, before an e- 

 ruption, by Vae elastic fluids, and falls and mixes wit]i 

 the cinders. The presence of Scoria in extinct volcan- 

 oes is admitted by most Kcptunians as a proof of the 

 action of fire, but its presence is, from the nature of 

 the substance, not of long duration; tlie rain-water car- 

 ries it off, and scatters it over the lower places and val- 

 lies, where the operation of time reduces it to an excel- 

 lent rich soil, when it of course loses all marks of vol- 



Time, with the assistance of heat an<i moisture, de- 

 composes and changes all the distinguishing marks of 

 rocks of Volcanic origin, and recomposes them into the 

 form and structure of rocks of Neptunian origin; but the 

 morcfrequently rocks of Neptunian origin are dccompa- 

 sed and recomposed, the stronger are the characters of 

 their origin, so that we cannot be deceived by the pre- 

 sent appearance of Neptunian rocks, wiien tracing 

 them up to their original form; but we are liable to 

 mistake rocks that were originally of Volcanic origin 

 before their decomposition, for rocks of the Neptunian, 

 after the change which time and the elements have 

 made in them. It is easy to conceive a large iieid of 

 Volcanic rocivs totally reduced to N eptuuiaa by tlie 



