b 



THE VOLCANIC ROCKS OF ICELAND. 65 



able, the rock assumes the character of the porous basaltic amyg- 

 daloid conglomerates, which are so frequently met with in Ice- 

 land as intermediate members between the palagonitic and 

 pyroxenic rocks. The darker ferruginous matrix, which under 

 the microscope has entirely the appearance of green-bottle glass, 

 but when examined by the naked eye in large pieces appears 

 more to resemble certain conglomeritic pyroxenic rocks, presents 

 sjiheroidal cavities with smooth surfaces, either empty or filled 

 wath granules of the non-ferruginous silicate. When this latter 

 crystalline mass, consisting of zeolitic substance, does not com- 

 pletely fill the cavities, druses of zeolitic crystals have been 

 formed, or isolated zeolitic crystals. 



This separation into ferruginous and non-ferruginous silicates 

 may be effected artificially in the palagonite and palagonitic 

 tuffs by the most simple means. When pieces of these sub- 

 stances about the size of hazel-nuts are rapidly heated in the 

 flame of a spirit-lamp, or before the blow-pipe, until they are 

 red-hot externally, all the phases of this metamorphism may 

 be most distinctly traced by means of a microscope of forty-fold 

 magnifying power, from the exterior vitrified crust to the interior 

 scarcely decomposed nucleus. In one zone, which, even by its 

 vitrified state, presents the most evident indications of previous 

 strong ignition, there may frequently be recognized a mass filled 

 with amygdaloid and drusy cavities which resembles most per- 

 fectly the basaltic amygdaloid rocks which dip beneath the trap 

 on the Esja near Hruni, and at numerous other parts of Iceland. 

 This resemblance extends so far, that even the crusts covering 

 these artificial crystal druses are identical in outward appearance 

 with those of the natural rocks. Even the manner in which the 

 crystals are situated upon the walls of the druses is precisely the 

 same. Sometimes brilliant chabasite crystals, with the striation 

 peculiar to this mineral, are observed in the ignited mass sepa- 

 rated from the uncrystalline matrix by a crystalline mass of 

 chabasite, followed by a crust resembling a saalband. 



The extraordinary abundance of zeolitic amygdaloid rocks in 

 Iceland is accounted for in the most simple manner by these 

 experiments, for the conditions requisite for their formation 

 could scarcely be found anywhere combined in a manner more 

 favourable than they are there. Even a hasty glance at the 



SCIEN. MEM.^Nat, Phil. Vol. I. Part I. F 



