Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/421

 former bringing before us the results of that action as shown at the surface, the latter revealing to us, as no modern volcano can do, some of those features of the action which go on below ground. It will be found, moreover, that between the rocks of each series there is, on the whole, a well-marked petrographical difference. The same species of rock is sometimes found indifferently in either division ; but when this occurs, as in the case of the dolerites and basalts, we often learn by practice to discover many little points of distinction, which, when combined, serve to give us a tolerably distinctive type for each of the two great series.

In both of these two leading divisions the rocks occur either as Crystalline or Fragmented. In the former section are included all the rocks which, like lavas, have been ejected in a melted state ; in the latter those which have been thrown out, like ashes and scoriae, in a fragmentary form.

The Crystalline Interbedded Rocks occur in the form of sheets or flows, either singly or in consecutive series ; they are, in short, old lava-flows, and present the same general structural and textural varieties as modern lavas show.

The Fragmental Interbedded Rocks likewise occur in sheets, or beds or layers ; they are the consolidated tuffs, conglomerates, and breccias arising from the ejection and deposition of ancient volcanic ashes and scoriae.

In the case of the Crystalline Intrusive Rocks I have found the simplest classification to be one based upon the form of the space into which these rocks were intruded and in which they consolidated. Accordingly, I have classed them as 1. Amorphous masses, which have been thrust through irregular fractures, and show in consequence no parallel bounding surfaces ; the syenites of Skye and Raasay are good examples. 2. Sheets, which were thrust between the bedding-planes of older rocks, and which differ from the sheets of the Contemporaneous Crystalline section in altering the beds above them, in showing none of the characteristic slaggy upper and under surfaces found in the contemporaneous flows, and in having some well-marked lithological differences, such as absence of amygdaloidal texture and greater compactness of grain towards the line of contact with the bounding surfaces of other rocks. 3. Dykes and Veins. These have resulted from the injection of melted rock along fissures. When the fissure was more or less vertical and straight, the intruded melted rock formed a Dyke ; when the crack was on a smaller scale and ran irregularly or branched, either vertically, horizontally, or at any angle, the result was a Vein or series of Veins.

4. In some cases the original orifices remain, which served as the vents by which the volcanic rocks were erupted to the surface. These volcanic pipes are now filled with various kinds of volcanic materials, and are termed Necks.

The Fragmental Intrusive Rocks only occur as Necks or as Veins connected with necks. They consist of agglomerate and tuff, sometimes exceedingly coarse and unstratified, composed of fragments of crystalline volcanic rocks, older tuffs, or of the surrounding strata through which the neck has been blown out.