1. Introduction .—The basaltic rocks of the north of England occur in two forms—as sheets lying amongst the sedimentary strata, and as dykes cutting through them. The intrusive character of the latter is, of course, undisputed; but there is much uncertainty in the minds of many geologists as to the character of the former. In this paper we purpose to show, from the detailed work of the Geological Survey, that in Northumberland there can be no doubt whatever as to the intrusive nature of the bed or beds of basalt known as the Whin Sill. This conclusion can be established both by the evidence of individual sections and by the line of outcrop of the trap. The question of the nature of the Whin Sill is of importance in three ways:—1st, as a point of theoretical geology, bearing upon the history of volcanic action in Britain; 2nd, in reference to the classification of the Carboniferous System; 3rd, in its bearing upon practical mining. The Whin Sill is best known in Teesdale, especially in the two fine waterfalls of High Force and Cauldron Snout. It also appears along the face of the Penine escarpment, and is beautifully exposed in the “Nicks” which furrow the face of that range. Although this district is known to us, we will not stay to describe it, because it has already much engaged the attention of geologists, especially Professors Sedgwick and Phillips. It is generally supposed that there is but little evidence of intrusion there; but this