85 



from one of the great centres of eruption (Barrow Hill), were produced 

 contemporaneously with its elevation ; since the basaltic matter which 

 flows laterally in apparent beds, follows the line of fault, bending and 

 leaping up, as it were, without a break from lower to higher levels. 

 From this fact Mr. Blackwell infers, that the basaltic matter must 

 have been in fusion when it was extruded laterally along the sur- 

 faces of certain strata, but that the subsequent dislocation by which 

 these beds were moved to diiferent levels, took place before the 

 igneous matter had cooled and when it was still plastic. This very 

 remarkable phsenomenon, which is indeed similar to examples cited 

 by Dr. IMac CuUoch, is an exception to the cases in other parts of the 

 district (Wolverhampton), where the beds of basalt are broken off and 

 change their relative places with the coal strata ; and thus we learn 

 that in the same district the faults were not all produced at one period. 



Near Wolverhampton, the underground trap, which is spread over 

 an area of five miles by two, is to a great extent perfectly conform- 

 able to the coal-measures above and below it ; but when traced for 

 upwards of three miles, its nonconformity to the coal-beds becomes 

 apparent, and this is further confirmed by the giving off of white 

 vertical felspathic dykes, similar to those before alluded to. It is, 

 however, worthy of record, that no centre of eruption has yet been 

 discovered in this part of the district whence the flow of basaltic 

 matter could have been derived, though it is believed that it may 

 have proceeded from the distant Rowley Hills. 



After minutely elucidating the distinctions and the variations of 

 structure and form in the various trappean rocks of this district, the 

 report proceeds to point out the changes which have been produced 

 in the coal-measures by their intrusion. The same beds of coal, which 

 in parts of the field exempt from basalt are highly bituminous, are, 

 when in contact or in the vicinity of that rock, either converted into 

 anthracite or charred into cinders. In the Wolverhampton tract the 

 upper portion of the coal beneath the greenstone is entirely cut out, 

 whilst the lower part is converted into anthracite, and often fissured 

 by vertical joints, accompanied by veins of calcareous spar; as if, 

 in driving off the bituminous matter of the coal, the extreme heat 

 had also occasioned a contraction of the carbonaceous mass. Be- 

 sides these alterations of the coal, the beds of ironstone are equally 

 affected, the clay being i^endered porcellaneous, and the sandstones, 

 usually much hardened, are in some cases even vitrified. 



In addition to these changes, which are analogous to effects ob- 

 served in other countries, and are such as a high degree of tempera- 

 ture will readily explain, other alterations are cited, with some of 

 which, indeed, Mr. W. Matthews acquainted me when I examined the 

 tract, and the solution of which is not so easy ; viz. that beds slightly 

 impregnated with iron at a distance from them, become gradually 

 more charged with it as they approach the igneous rocks, and are 

 of very superior quality in their immediate vicinity. This fact, in- 

 deed, is perfectly in accordance with what I have lately observed in 

 the Ural Mountains, where masses of iron ore are crystalline and 

 even highly magnetic when in contact with eruptive rocks. 



