Ch. XXVIII.] PALAGONITE TUFF— AGGLOMERATE— LATERITE. 603 



pressed, as nmcli as ordinary trap rocks, and to be often identical 

 with these in mineral composition. 



Palagoniie Tuff.— -The nature of volcanic tnffs must vary according 

 to the mineral composition of the ashes and cinders thrown out of each 

 vent, or from the same vent, at different times. In descriptions of 

 Iceland, we read of Palagonite tuffs as very common. The name Pa- 

 lagonite was first given by Prof. Bnnsen to a mineral occurring in the 

 volcanic formations of Palagonia, in 'Sicily. It is rather a mineral 

 substance than a mineral, as it is always amorphous, and has never 

 been found crystallized. Its composition is variable, but it may be de- 

 fined as a hydrosilicate of alumina, containing oxide of iron, lime, 

 magnesia, and some alkali. It is of a brown or blackish-brown color, 

 and its specific density, 2 # 43. It enters largely into the composition 

 of volcanic tufts and breccias, and is considered by Bunsen as an 

 altered rock, resulting from the action of steam on volcanic tuffs. 



Agglomerate. — In the neighborhood of volcanic vents, we frequently 

 observe accumulations of angular fragments of rock, formed during 

 eruptions by the explosive action of steam, which shatters the subja- 

 cent stony formations, and hurls them up into the air. They then fall 

 in showers around the cone or crater, or may be spread for some dis- 

 tance over the surrounding country. The fragments consist usually of 

 different varieties of scoriaceous and compact lavas ; but other kinds 

 of rock, such as granite or even fossiliferous limestones, may be inter- 

 mixed; in short, any substance through which the expansive gases 

 have forced their way. The dispersion of such materials may be 

 aided by the wind, as it varies in direction or intensity, and by the 

 slope of the cone down which they roll, or by floods of rain, which 

 often accompany eruptions. But if the power of running water, or 

 of the waves and currents of the sea, be sufficient to carry the frag- 

 ments to a distance, it can scarcely fail (unless where ice intervenes) 

 to wear off their angles, and the formation then becomes a conglom- 

 erate. If occasionally' globular pieces of scoriae abound in an agglom- 

 erate, they do not owe their round form to attrition. 



The size of the angular stones in some agglomerates is enormous ; 

 for they may be two or three yards in diameter. The mass is often 

 50 or 100 feet thick, without showing any marks of stratification. 

 The term volcanic breccia may be restricted to those tuffs which are 

 made up of small angular pieces of rock. 



The slaggy crust of a stream of lava will often, while yet in mo- 

 tion, split up into angular pieces, some of which, after the current has 

 ceased to flow, may be seen to stick up five or six feet above the gen- 

 eral surface. Such broken-up crusts resemble closely in structure the 

 agglomerate above described, although the composition of the mate- 

 rials will usually be more homogeneous. 



Laterite is a red or brick-like rock composed of silicate of alumina 

 and oxide of iron. The red layers, called " ochre beds," dividing the 

 lavas of the Giant's Causeway, are laterites. These were found by 



