654 



GEOLOGY. 



which exceed in grandeur, and in picturesque effect those of Staffa. " The promontory," 

 says Dr. Macculloch, " rests on a bed of compact grey limestone, approaching to a stone 

 marl. This bed, which is three or four feet thick, rests on a still lower bed, of hard 

 reddish stone. Masses of bitumenised wood, penetrated with carbonate of lime, are found 

 in the marl stratum, not at all flattened. Portions of trunks of trees, retaining their 

 original shape, but petrified, are found in the same stratum ; the rifts are filled with 

 chalcedony, approaching in aspect to semi-opal. The columns on this island are both 

 perpendicular and inclined, and some of them are bent or curved." They vary from two 

 feet to a few inches in diameter. 



The varieties of basalt are frequently associated with and pass into each other. Where 

 the columnar structure is not distinctly defined, there is often a tendency to it plainly 

 developed, as in the basaltic hills near Dudley ; while many basalts, which are non- 

 columnar, upon a section being made, or decomposition 

 taking place, exhibit a globular construction, as in fig. 4., 

 similar to that of a number of hard balls imbedded in a 

 softer cementing mass. The most important example of 

 the occurrence of basalt in dykes is the great Cleveland 

 basaltic wall, which cuts through various strata the coal 

 measures, red sandstone, and lias in its course of from 



fifty to sixty miles, extending from the west side of the county of Durham, across the 

 Tees, and through the Cleveland Hills in the East Riding of Yorkshire, to the sea be- 

 tween Scarborough and Whitby, probably entering the bed of the German Ocean. " The 

 effects of this basaltic dyke on the different rocks through which it passes are truly de- 

 serving notice. When it comes in contact with limestone, the limestone is often found 

 granular and crystalline. When it crosses the coal strata, and comes in contact with seams 

 of coal, the substance of the coal is for several feet converted into soot. At a greater dis- 

 tance from the basalt the coal is reduced to coke or cinder, which burns without smoke, 

 and with a clear and durable heat. At the distance of fifty feet from the dyke, the coal 

 is found in its natural unaltered state. The formation of basaltic dykes is sufficiently 

 explained by what takes place in the vicinity of volcanoes. Before the confined vapour 

 that afterwards issues through the crater finds a vent there, the surface of the ground in 

 the vicinity of the volcano is frequently upheaved, and fissures of great extent are made, 

 into which melted lava is sometimes forced, which, on crossing, forms a wall or dyke, in 

 every respect similar to a basaltic dyke. During an eruption of Vesuvius that took place 

 in 1794, a vent of this kind was formed near the bottom of the mountain, 2375 feet in 

 length and 237 feet in breadth, which became filled with compact lava. Rents or fissures, 

 of some miles in length, have been opened on the sides of Etna." 



Porphyries. It has been previously observed that the term porphyry is not now 

 employed to designate any particular kind of rock, but to denote all rocks which have a 

 homogeneous, compact, or earthy base, through which are disseminated crystalline masses 

 of some other mineral of contemporaneous origin with the base. It refers, therefore, to a 

 distinctive mechanical structure, and not to any peculiar mineralogical composition. The 

 substance that forms the base of the rock gives the name to the porphyry. Hence we 

 may have 



Felspar porphyry base of felspar, with crystals of quartz, or mica. 

 Greenstone porphyry base of greenstone, with crystals of felspar, quartz, or calcareous 

 spar. 



Claystone porphyry base of claystone, with crystals of felspar. 



Pitchstone porphyry base of pitchstone, with crystals of felspar. 



Porphyry, in its several varieties, occurs in amorphous masses and in veins ; and frequently 



