OBSERVATIONS ON TRAP ROCKS. 157 



sometimes inclining to splintery, and sometimes flatly con- 

 clioidal ; and it is pei lectly opake. These rocks have moul- 

 dered so much, that their original form cannot he ascer- 

 tained ; the shape in whicii lliey now appear undoubtedly 

 being the result of decomposition ; but the particular sj)ecies 

 of which 1 am now speaking, is usually found in spheroids 

 formed of concentric crusts that fall off in succession as the 

 mass decays. After the first coat is detached, tiiese balls 

 appear perfectly sound ; but on being broken, are found to 

 be enveloped with two or three other coats, in a progres- 

 sive state of preparation for falling off', which decreases to- 

 wards the centre till the mass becomes thoroughly sound. 

 The outermost ciust, by shrinking and chapping, is tilled 

 with an inlinite number of fissures which cross each other 

 in every direction, and disposed, on being completely de- 

 tached, to fall into very small pieces, that are soon entirely 

 disintegrated. Within the itmermost perceptible layer, the 

 mass, wliich never contains any foreign body as a nucleus, 

 is equally haid to its centre. In a single instance, I found 

 a quartz pebble, rounded by attrition, embedded in one of 

 these rocks ; but they disclose neither organic remains nor 

 vegetable impressions. What shews that the spheroids were 

 not originally formed on a nucleus, or that they did not at 

 first take a figurate form, is, that when, as sometimes hap- 

 pens, they are split by exposure to the weather, each part 

 assumes, in the progress of further decomposition, the foi m 

 of a distinct sphere, whose crusts take a new point for their 

 common centre, witliout regard to that by which the general 

 exfoliation before proceeded. In truth, I am of o|)inion tliat 

 basalt, or greenstone, is originally always amorphous, and 

 that it takes a determinate form only in a state of decom- 

 position ; as is shewn by the columnar basalt of t!ie Giant's 

 Causewaii in Jrdand ; of t!ic lake o[ Bulsonna in [fahf ; of 

 IlaUfbc.rg a!id Iliinnchersf in Sweden ; and other places ; 

 which exhibits regular prismatic forms onlv when it has long 

 been exposed to the action of the atmosphere ; lor when- 

 ever a part of the surliice has l)ccn removed, the interior 



