574 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
intrusive igneous material. In the Scottish coal-fields sheets of basalt 
have been forced along the surfaces of coal-seams, and even along their 
centre so as to form a bed or sheet in the middle of thecoal. The coal in 
these cases is sometimes beautifully columnar, its slender hexagonal and 
pentagonal prisms, like rows of stout pencils, diverging from the surface 
of the intrusive sheet.! 
Other examples of the production of this structure have been described 
in dolomite altered by quartz-porphyry (Campiglia, Tuscany); fresh- 
water limestone altered by basalt (Gergovia, Auvergne); basalt-tuff and 
granite altered by basalt? (Mt. Saimt-Michel, Le Puy). 

Fic. 297,—SANDSTONE (a, @) RENDERED PRISMATIC BY DOLERITE (6, b), BIsHOPBRIGGS, 
GLASGow. 
Calcination, Melting, Coking.*—By the great heat of erupted 
masses, more especially of basalt and its allies, some rocks have 
undergone partial fusion, their matrix or some of their component 
minerals having been melted, while others have been entirely fused. 
Among granite fragments ejected with the slags of old volcanic vents in 
Auvergne, some present no trace of alteration, others are burnt as if 
they had been in a furnace, or are partially melted so as to look like 
slags, each of their component minerals, however, remaining distinct. 
In the Eifel volcanic region, the fragments of mica-schist and gneiss 
ejected with the volcanic detritus have sometimes a crust or glaze of 
glass. Sandstones, though most frequently baked into a compact 
quartzite, are sometimes changed into an enamel-like mass in which, 
* Coal and lignite, with their accompanying clays, altered by basalt, diabase, mela- 
phyre, &c., Ayrshire, Scotland; St. Saturnin, Auvergne; Meissner, Hesse Cassel ; 
Ettingshausen, Vogelsgebirge ; Sulzbach, Upper Palatinate of Bavaria: Fiinfkirchen, 
Hungary: by trachyte, Commentry, Central France; by phonolite, Northern Bavaria. 
2 Naumann, “ Geognosie,” i. p. 737. 
§ It is worthy of observation that changes of the kind here referred to occur most 
commonly with basalt-rocks, melaphyres, and diabases. Trachyte has been a less 
frequent agent of alteration, though some remarkable examples of its influence have been 
noted. Poulett Scrope (Geol. Trans. 2nd Ser. II.) describes the alteration of a 
trachyte conglomerate by trachyte into a vitreous mass. Quartz-porphyry and diorite 
occasionally present examples of calcination, or more or less complete fusion. But with 
the granitic and syenitic rocks changes of this kind have never been observed. Naumann, 
** Geognosie,”’ i. p. 744. 
