Ch. XXVHI.] AGGLOMERATE — LATEEITE. 471 



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 subjacent 

 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 distance 

 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 intermixed ; 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 suffi- 

 cient to carry the fragments to a distance, it can scarcely fail (unless 

 where ice intervenes) to wear off their angles, and ihe formation then 

 becomes a conglomerate. If occasionally globular pieces of scoriae 

 abound in an agglomerate, they do not owe their rounded form to at- 

 trition. 



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 motion, 

 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 general surface. 

 Such broken-up crusts resemble closely in structure the agglomerates 

 above described, although the composition of the materials will usually 

 be more homogeneous. 



Laterite is a red, jaspery, 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 

 Delesse to be trap impregnated with the red oxide of iron, and in part 

 reduced to kaolin. When still more decomposed, they were found to be 

 clay colored by red ochre. As two of the lavas of the Giant's Causeway 

 are parted by a bed of lignite, it is not improbable that the layers of 

 laterite seen in the Antrim cliffs resulted from atmospheric decomposi- 

 tion. In Madeira and the Canary Islands, streams of lava of subaerial 

 origin are often divided by red bands of laterite, probably ancient soils 

 formed by the decomposition of the surfaces of lava-currents, many of 

 these soils having been colored red in the atmosphere by oxide of iron, 

 others burnt into a red brick by the overflowing of heated lavas. These 

 red bands are sometimes prismatic, the small prisms being at right angles 

 to the sheets of lava. Red clay or red marl, formed as above stated by 

 the disintegration of lava, scoria?, or tuff, has often accumulated to a 

 great thickness in the valleys of Madeira, being washed into them by 

 alluvial action ; and some of the thick beds of laterite in India may have 



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