[447] Dr. Stecher, Tschermak's Mineralog. Mittheil. vol. ix. (1887) p. 193. Proc. Roy. Soc. Edin. vol. xv. (1888) p. 162.
Of the ordinary and characteristic dolerites without olivine which constitute most of the intrusive masses, the various types enumerated in the tabular arrangement are abundantly developed in Central Scotland. Thus the normal ophitic type is displayed by the uppermost sill of the Burntisland series, and by the rock which forms the plug of the Binns Hill neck in Linlithgowshire. The Ratho type is well seen in the large sill at Ratho, likewise in the extensive intrusive sheets in the west of Linlithgowshire as at Muckraw and Carribber. The Burntisland sill type is shown by the lower sills of Burntisland and by some others in the same region, especially by that of Colinswell, and by another on the shore east from the Poorhouse, near Kinghorn. The great boss among the Bathgate Hills likewise displays it. The Bowden Hill type occurs in well-marked development at Bowden Hill, three miles south-west of Linlithgow, and in the massive sill at St. Margaret's, west from North Queensferry.
The non-olivine-bearing basalts are found in various bosses and sheets in the basin of the Firth of Forth. Thus the Binny Craig type occurs in the prominent and picturesque sill from which it is named, likewise among the intrusive sheets near Kirkcaldy, in Fife. Sometimes the same mass of rock displays more than one type of structure, as in the case of the great Galabraes neck among the Bathgate Hills wherein both the Tholeiite and Burntisland sill types may be recognized.
Some of the sills in West Lothian, as I pointed out many years ago, contain bitumen and give off a bituminous odour when freshly broken. They have been injected into bituminous shales or coal-seams.[448]
[448] Geol. Survey Memoir on Geology of Edinburgh (Sheet 32, Scotland), p. 46.
2. Andesites.—Rocks referable to this series appear to have been of rare occurrence among the puy-eruptions. Examples of them containing as much as 60 per cent of silica occur among the lavas of the Limerick basin. Some of the necks and what may be sills in the same district likewise consist of them.
3. Trachytes and Quartz-bearing Rocks.—Acid rocks, as I have already said, are extremely rare among the puy-eruptions. The only important examples known to me are those around the Limerick basin, where they rise apparently in old vents and form conspicuous rounded or conical hills. These rocks have been examined microscopically by Mr. W. W. Watts. One of the most interesting varieties, which occurs at the Standing Stone near Oola, was found by him to show quartz enclosing ophitically the felspars which, with well-terminated prisms, project into it. Further west, near Knockaunavoher, another boss occurs with conspicuous quartz. These rocks have much in common with trachytes but have a wholly crystalline structure. They will be described in the account of the Limerick basin.
B. Tuffs
The fragmental rocks connected with the puy-eruptions form a well-marked group, easily distinguishable, for the most part, from the tuffs of the plateaux. They vary from exceedingly fine compacted dust or volcanic mud, through various stages of increasing coarseness of texture, to basalt-conglomerates and tumultuous agglomerates.
The fragmentary material found in the necks of the puys is generally an agglomerate of a dull dirty-green colour. The matrix ranges from a fine compact volcanic mud to a thoroughly granular detritus, and sometimes shows a spheroidal concentric structure in weathering. In this matrix the lapilli are distributed with great irregularity and in constantly varying proportions. They consist in large measure of a pale yellowish-green, sometimes pale grey, very basic, finely vesicular, devitrified glass, which is generally much decomposed and cuts easily with the knife. This highly basic substance is a kind of palagonite. So minute are its vesicles that under the microscope a thin slice may present a delicate lace-like network of connected walls, the palagonite occupying much less space than the vesicles. The material has been a finely frothed-up pumice.