166 Geology of Okehampton, %c. Devon. [Sept. 



-glass more or less slaggy, sometimes into a dirty-white enamel 

 more or less mixed with black patches. The criterion, there- 

 fore, proposed by Cordier, here fails us.* A portion of the rock 

 broken into small fragments, and exposed for an hour to the 

 heat of a Black's furnace, gave a black glass much resembling 

 that produced by various forms of the dolerite under the same 

 circumstances. 



The same obscurity which is attached to the mineralogical 

 character of this rock seems to extend in some measure to its 

 relations with the conglomerate in which it occurs. In some 

 places it covers, and in others is covered by sandstone. On the 

 road from Killerton to Silverton, near a house occupied fin the 

 year 1812) by Mrs. Brown, we saw it resting on the large 

 grained conglomerate ; and at one of the Radden quarries, near 

 Thorverton, covered by a sandstone bed of from three to ten. 

 feet in thickness. Its line of separation from the sandstone is 

 sometimes tolerably distinct. In one quarry at Thorverton, a 

 line of sandy clay, not quite a foot thick, prevents their actual 

 contact. At other places, especially at the Radden Quarries, 

 the two substances appear to pass so insensibly into each othe7 

 as to induce for the moment a conjecture that both were the 

 result of a common deposition modified in its characters by the 

 partial intrusion of some extraneous matter. This phenomenon 

 has already been noticed by Mr. Greenough. " What mineralo- 

 gist," he asks, " can draw a line of demarcation between the 

 red marl and the toadstone at Heavitree." (Essay, p. 215). Your 

 geological readers have probably already anticipated that a vul- 

 canist would at once decide that the whole of the amygdaloidal 

 beds was a series of whyn-di/kes ; while others will be disposed 

 to regard them as concretions or depositions more nearly con- 

 nected and contemporaneous with the strata which envelope 

 them. The difficulty would probably vanish before a more accu- 

 rate investigation of their character and position, which I beg 

 to recommend to such mineralogists as may travel westward. 



It may be added that at the Radden Quarries we noticed the 

 occasional tendency of this rock to split into basaltiform balls ; 

 and in one spot observed it traversed by nearly horizontal veins 

 of its own substance differing slightly from the mass by their 

 greater compactness, and the largeness of the nodules which 

 they contained. The veins of extraneous matter were mostly 

 vertical, or at a very high angle. 



* I am acquainted with the experiments of M. Cordier only through the notice given 

 cf them in M . Bruce's Geologie de l'Ecosse. 



