50 M. Molis' Summary of Geognostical Phenomena- 

 Trap Dykes. — Another circumstance which deserves atten- 

 tion is, that rocky masses, which, in other respects lie in regu- 

 lar beds, under no unusual inclination, and in regular sequence 

 over and under one another, are bounded by vertical or more 

 or less inclined planes, so that their strata, or the tabular masses 

 into which they are separated, meet with their heads, as it is 

 generally expressed. This phenomenon is explained, and with 

 greater appearance of probability than in the so-called primi- 

 tive and transition rocks, by means of great dislocations ; and 

 the proof of the correctness of this explanation is found in this, 

 that, at the boundary, a substance is met with different from the 

 rocks of both the mountain-masses, and which, from its form and 

 other characters, has the aspect of having been formed in a 

 fissure which had been produced after the existence of these 

 masses, and which had been opened previous to the appearance 

 of the separating mass. Greenstones and basalts more espe- 

 cially, but likewise rocks which are regarded as of mechanical 

 origin, occupy these fissures. However hasty it would be to 

 reject this explanation, yet, on the other hand, it must like- 

 wise be remarked, that there are instances of this kind in which 

 there is no rock present, separating the rocky masses at their 

 meeting, and even no fissure or distinct concretion-surface, but 

 in which the two masses actually are in direct contact, are 

 firmly united with each other, and even so pass into each other, 

 and become mutually confounded, that we are not able to 

 assign a fixed boundary between them ; and that appearances 

 present themselves which are less for than against this expla- 

 nation. In the neighbourhood of Newcastle, it has been ob- 

 served, that some of the basalt or greenstone veins (whin- 

 dykes), notwithstanding their thickness (amounting to as much 

 as seven fathoms), do not traverse the beds of coal with which 

 they come in contact ; and Mr Buddie, one of the most intelli- 

 gent of the engineers of the coal-mines, remarks, that these 

 and other facts are to be regarded as exceptions from the gene- 

 rally received opinion, " that the whin-dykes are formed by 

 the eruption of basalt from fissures, and that they descend to 

 an unascertained depth." " It is also doubtful," 1 he adds, " if 

 the basaltic dykes which traverse the coal-fields of Newcastle 



