GIESECKE ON THE MINERALOGY OF DISKO. 337 



large masses of columnar basalt, resting on gneiss ; but beyond 

 this place, tliere is no farther approach, the country being 

 covered by the Great Boreal Glacier — the Northern Iceblink, 



The direction of the trap-rocks, which are here spread over 

 such an extent of country, is almost entirely similar, being 

 nearly horizontal, stretching from south-west to north-east. 

 The beds of which they are composed are of a very unequal 

 thickness ; those of basalt are most prevalent. The hills com- 

 posed of gneiss and granite are never highly elevated; and 

 the Floetz rocks are placed immediately on the gneiss, which 

 is always slightly decomposed upon the surface, where in con- 

 tact with the trap. The prismatic basalt of this district, as of 

 that species distinguished in "Germany by the name of Ba- 

 saltic Greenstone (griinsteinartiger Basalt). It is almost pure, 

 but sometimes contains a few detached specks, perhaps crystals, 

 of felspar. I found only in one place some small grains of 

 augite and of hornblende. The massive basalt, on the con- 

 trary, often becomes amygdaloidal by the small globules of 

 mesotype, stilbite, and quartz which it contains. It occurs 

 very generally undermost, and touching the Primitive rocks, 

 which is very rarely the case with the columnar basalt. 



The trap-tuff, which is very common among the Floetz rocks 

 of Disko, rests also always immediately on the Primitive rocks ; 

 indeed, I never found it in any other situation in that island. It 

 appears to me here necessary to mark two varieties of this rock, 

 namely, that which consists almost entirely of fragments of wacke 

 contained in a paste of the same substance in a state of decom- 

 position ; it is of a very fine grain, very soft, and almost friable. 

 The other is composed of fragments of wacke, but more compact, 

 and of globular pieces of basalt. When these globules are 

 broken, the interior is occupied by geodes of crystallised apophyl- 

 lite, accompanied with capillary mesotype, sometimes decomposed 

 and reduced to powder, in which state it is known by the name 

 of earthy zeolite. These are the only minerals 1 found in this 

 globular basalt. The apophyllite I never observed in the other 

 variety of trap-tuff, in which I discovered no simple mineral 

 whatever, except some very small geodes of radiated zeolite. I 

 shall distinguish the one by the name of Trap-tuff, and the other 

 by that of Basalt-tufF. The last appears to me to be the oldest of 

 the two, and occurs, wherever I saw it, under the other. If the 

 tuff be entirely absent, then the amorphous basalt occupies its 

 place ; and on it rests the amygdaloid, the paste of which is of a 

 reddish-brown colour. It is the amygdaloid of this colour in 

 which the greatest number of minerals occur, such as stilbite, 

 mesotype, quartz, calcedony, and igloite. When exposed to the 

 action of the weather, this rock becomes extremely fragile, and 

 falls in conchoidal fragments, almost like bole. It occasions, 

 particularly in the spring season, by reason of its feeble cohesion, 

 immense devastation. Rent by the effects of the severe frosts of 

 winter, it falls in huge blocks into the valleys, when the basalt, 

 deprived of its support, is precipitated in enormous masses, and to 

 36122. V 



