Chap. VIII.] trap dykes and intrusions. 147 



most irregular manner, altering and hardening it, and at times causing 



it to assume a perfectly columnar structure. Apparently the coal has 



been fused by the trap, and the latter has been driven into the seam, 



wherever it could force itself a passage. 



The almost universal parallelism of these dykes, with the beds 



among which they occur, almost of necessity 



Not contemporaneous. . ' J 



induces the belief, at first sight, that they are 



interstratified lava flows of contemporaneous age, like those of the 

 Rajmahal Hills or of the Deccan. More close examination, however, 

 shows that the rocks, both above and below, are altered by them, and 

 that if traced for any distance, they, in general, cut across the strata. 

 But, in coal seams, there can be no question, from their irregularly 

 intrusive character, that they are of posterior and not contem- 

 poraneous formation. And in sandstones, they are occasionally seen 

 to split up and anastomose, in a manner which can only be due to 

 intrusion. 



These dykes (for they are dykes, although horizontal and not 

 vertical) are, as already remarked, almost confined to the Lower Damti- 

 das. A few instances, however, occur in the Raniganj series. One 

 of these is seen stretching from the central branch of the JMunia, South 



Principally in Lower of Madanmonpur, to near Sripur ; another occurs 

 DamMa group. about ji mileg f^t^ Eastj near Kaithi, and a 



few instances of coal, intersected by trap, have also been referred to • 

 that near Chalwidi, on the Nunia, "West of Raniganj, and of the seam 

 just below that worked at Mainanagar being examples. But these are 

 singularly rare, when compared with the great prevalence of such 

 horizontal trap intrusions among the sandstones, and far more conspi- 

 cuously among the coals, of the Lower Damtidas. So numerous were 

 these little irregular dykes in that group of beds that it was found 

 impracticable to map any, except the most important and conspicuous. 

 It is worthy of remark that the neighborhood of Samdi and of the 



