Parr VII. § 2] LOCAL METAMORPHISM. 575 
when the rock consists of an argillaceous or calcareous matrix with 
dispersed quartz-grains, the infusible quartz may be recognized 
(Oberellenbach, Lower Hesse). According to Bunsen’s observations, 
voleanic tuff and phonolite have sometimes been melted for several 
feet on the sides of the dolerite dykes which traverse them, so as to 
present the aspect of pitchstone or obsidian.’ Besides complete fusion 
and fluxion structure there has sometimes been also a production of 
microscopic crystallites in the fused portions resembling those of 
eruptive rocks. | 
The effects of eruptive rocks upon carbonaceous beds and 
particularly upon coal-seams are among the most conspicuous 
examples of this kind of alteration. They vary considerably, 
according to the bulk and nature of the eruptive sheet, the thickness, 
composition, and structure of the coal-seam, and probably other 
causes. In some cases the coal has been fused and has acquired a 
blistered or vesicular texture, the gas cavities being either empty or 
filled with some infiltrated mineral, especially calcite (east of Fife). 
In other examples the coal has become a hard and brittle kind of 
anthracite or “blind coal,” owing to the loss of its more volatile 
portions (west of Fife). This change may be observed in a coal- 
seam six or eight feet thick, even at a distance of 50 yards from a 
large dyke. Traced nearer to the eruptive mass the coal passes into 
a kind of pyritous cinder scarcely half the original thickness of the 
seam. At the actual contact with the dyke it becomes by degrees a 
kind of caked soot, not more perhaps than a few inches thick (South 
Staffordshire, Ayrshire). Coal altered into a prismatic substance 
has been-above (p. 573) referred to; it has even been observed 
changed into graphite (New Cumnock, Ayrshire). 
The basalt of Meissner (Lower Hesse) overlies a thick stratum of 
brown coal which shows an interesting series of alterations. Immediately 
under the igneous rock a thin seam of impure earthy coal (‘“letten”) 
appears as if completely burnt. The next underlying stratum has been 
altered into metallic-lustred anthracite, passing downwards into various 
black glossy coals beneath which the brown coal is worthless. The depth 
to which the alteration extends is 5°3 metres.2, Another example of 
alteration has recently been described by G. vom Rath from Fiinfkirchen 
in Hungary. A coal-seam has there been invaded by a basic igneous 
rock (perhaps diabase) now so decomposed that its true lithological 
character cannot be satisfactorily determined. Here and there the 
intrusive rock lies concordantly with the stratification of the coal, in 
other places it sends out fingers, ramifies, abruptly ends off, or occurs 
in detached nodular fragments in the coal. ‘The latter in contact with 
the intrusive naterial is converted into prismatic coke. The analysis of 
three specimens of the coal throws light on the nature of the change. 
1 Usually the vitreous band at the margin of a dyke of basalt or dolerite is tachylitic, 
belonging to the intruded rock and not to that through which it has risen. 
2 Moesta, “ Geologische Schilderung, Meissner und Hirschberge,” Marburg, 1867. 
8 G. vom Rath, N. Jahrb. 1880, p. 276. In the above analysis the bitumen includes 
all volatile constituents driven off by heat, hence coke and bitumen = 100. 
