1871.] GEIKIE TERTIAEY VOLCANIC ROCKS. 281 



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 Fragmental. 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 

 floivs, 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 Eocks likewise occur in sTieets, 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 liocks 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 conse- 

 quence 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 amyg- 

 daloidal texture and greater compactness of grain towards the line 

 of contact with the bounding surfaces of other rocks. 3. DyTces 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, hori- 

 zontally, or at any angle, the result was a Yein 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, some- 

 times 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. 



