measures, and, on the East Lomond, with Mountain limestone.* Trap of the Old Red sandstone period. — By referring to the section explanatory of the structure of Forfarshire, already given (p. 99.), the reader will perceive that beds of conglomerate, No. 3., occur in the middle of the old red sandstone system, 1, 2, 3, 4. The pebbles in these conglomerates are sometimes composed of granitic and quartz rocks, sometimes exclusively of different varieties of trap, which, although purposely omitted in the above section, are often found, either intruding themselves in amorphous masses and dikes into the older fossiliferous tilestones, No. 4., or alternating with them in conformable beds. All the different divisions of the red sandstone, 1, 2, 3, 4, are occasionally intersected by dikes, but they are very rare in Nos. 1. and 2., the upper members of the group consisting of red shale and red sandstone. These phenomena, which occur at the foot of the Grampians, are repeated in the Sidlaw Hills; and it appears that in this part of Scotland volcanic eruptions were most frequent in the earlier part of the old red sandstone period. The trap rocks alluded to consist chiefly of felspathic porphyry and amygdaloid, the kernels of the latter being sometimes calcareous, often *Fleming MS. Part of this tract I have myself examined with Dr. F. chalcedonic, and forming beautiful agates. We meet also with claystone, clinkstone, greenstone, compact felspar, and tuff. Some of these rocks flowed as lavas over the bottom of the sea, and enveloped quartz pebbles which were lying there, so as to form conglomerates with a base of greenstone, as is seen in Lumley Den, in the Sidlaw Hills. On either side of the axis of this chain of hills (see section, p. 99.), the beds of massive trap, and the tuffs composed of volcanic sand and ashes, dip regularly to the south-east or northwest, conformably with the shales and sandstones. Dr. Fleming has observed similar trap rocks in the old red sandstone of northern Fifeshire, where they are covered immediately by the yellow sandstone which forms the base of the mountain limestone and coal-measures. Silurian period. It appears from the investigations of Mr. Murchison in Shropshire, that when the lower Silurian strata of that county were accumulating, there were frequent volcanic eruptions beneath the sea; and the ashes and scoriæ then ejected gave rise to a peculiar kind of tufaceous sandstone or grit, dissimilar to the other rocks of the Silurian series, and only observable in places where syenitic and other trap rocks protrude.* These tuffs occur on the flanks of the Wrekin and * Murchison, Silurian System, &c. p. 230. Caer Caradoc, and contain Silurian fossils, such as casts of encrinites, trilobites, and mollusca. Although fossiliferous, the stone resembles a sandy claystone of the trap family.* Thin layers of trap, only a few inches thick, alternate, in some parts of Shropshire and Montgomeryshire, with sedimentary strata of the lower Silurian system. This trap consists of slaty porphyry and granular felspar rock, the beds being traversed by joints like those in the associated sandstone, limestone, and shale, and having the same strike and dip. + In Radnorshire, there is an example of twelve bands of stratified trap alternating with Silurian schists and flagstones in a thickness of 350 feet. The bedded traps consist of felspar-porphyry, clinkstone, and other varieties; and the interposed Llandeilo flags are of sandstone and shale, with trilobites and graptolites. + Cambrian volcanic rocks. In Pembrokeshire stratified greenstone, felspar-rock, and a breccia containing fragments of trap, alternate conformably in thick parallel masses with regularly stratified sandstone and schist of the upper Cambrian system. These trappean masses, says Mr. Murchison, must have been evolved at intervals from volcanic fissures at the bottom of the * Murchison, Silurian System, &c. p. 230. sea, when the sand, pebbles, and mud, now forming the accompanying sedimentary rocks, were deposited.* Professor Sedgwick, in his account of the geology of Cumberland, has described various traprocks which accompany the green slates of the Cambrian system, beneath a limestone containing organic remains. Different felspathic and por phyritic rocks and greenstones occur, not only in dikes, but in conformable beds; and there is occasionally a passage from these igneous rocks to some of the green quartzose slates. Professor Sedgwick supposes these porphyries to have originated contemporaneously with the stratified chloritic slates, the materials of the slates having been supplied, in part at least, by submarine eruptions oftentimes repeated. + * Murchison, Silurian System, &c. p. 404. 487 CHAPTER XXIV. ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS. Difficulty in ascertaining the precise age of a plutonic rock Test of age by relative position Test by intrusion and alteration - - Test by mineral composition - Test by included fragments-Recent and Pliocene plutonic rocks, why invisible Tertiary plutonic rocks in the Andes Granite altering Cretaceous rocks - Granite altering Lias in the Alps and in Sky Granite of Dartmoor altering Carboniferous strata- Granite of the Old Red sandstone period Syenite altering Silurian strata in Norway— Blending of the same with gneiss Most ancient plutonic rocks - Granite protruded in a solid form On the pro - bable age of the granite of Arran, in Scotland. WHEN we adopt the igneous theory of granite, as explained in the 9th chapter, and believe that different plutonic rocks have originated at successive periods beneath the surface of the planet, we must be prepared to encounter greater difficulty in ascertaining the precise age of such rocks, than in the case of volcanic and fossiliferous formations. We must bear in mind, that the evidence of the age of each contemporaneous volcanic rock was derived, either from lavas poured out upon the ancient surface, whether in the sea or in the atmosphere, or from tuffs and conglomerates, also deposited at the surface, and either containing organic remains |