OUTLINES OF 1 ! OLOGV PART i 



teristic grouping of volcanic and sedimentary materials 

 in the Carboniferous Limestone series of central Scot- 

 land. In the diagram, Fig. 28, the series of rocks 

 marked P in the southern part of the map are examples 

 of interbedded lavas and tuffs. They occur under cir- 

 cumstances very similar to those of ordinary stratified 

 rocks. They dip one below the other in orderly succes- 

 sion, and are traversed by faults, like the beds of sand- 

 stone and conglomerate which lie below, between, and 

 above them. In an actual volcanic cone, where only 

 volcanic materials occur, a more complex arrangement 

 is found. Lavas and tuffs there succeed each other in 

 rapid alternations, often cut by dykes or veins. Volcanic 

 cones belonging to old geological periods have seldom 

 been preserved, for they would usually be washed down 

 by the waters of the seas or lakes in which they sank. 

 It is not uncommon, however, to meet with the " necks " 

 above described (Figs. 59, 60, 61), which may be re- 

 garded as the roots or stumps of cones from which the 

 overlying pile of ejected materials has been worn away 

 by denudation (V Fin Fig. 28). This lower or down- 

 ward prolongation of the original cone, as I have already 

 mentioned, may consist either of masses of lava or other 

 crystalline rock, or of consolidated fragmentary materials. 

 In the latter case the tuff or agglomerate has sometimes 

 become itself crystalline, owing, no doubt, to the long 

 continued upward passage of steam, hot vapours, and 

 gases through the volcanic vent after the explosions 

 ceased. 



As illustrations of the way in which the structure of a 

 volcanic region is worked out and expressed upon a map, 



