part 2] GEOLOGY OF THE MELD ON TALLETS. i>7 



There are now principally two : a pale-green actinolite, mainly 

 forming remarkably uniform areas which consist of felted di- 

 vergent prisms, rarely giving a section showing the characteristic 

 cleavage ; and a mica of warm sepia tint in felted particles of 

 minnte blades. These two minerals mingle but little. There 

 are fair-sized forms of a black iron-ore, probably ilmenite. A 

 pretty blue tourmaline is curiously associated with pyrite, parallel 

 needles of the tourmaline penetrating masses of pyrite, and thus 

 giving cross-sections of tourmaline divided by a network of the 

 other mineral. 



Conclusions as to the c Dark Igneous ' Rocks. 



A marked feature of the 'dark igneous' rocks is that they are 

 locally agglomeratic ; as such they have been identified by some 

 geologists as metamorphosed tuffs. But, on the other hand, 

 every exposure is also in part homogeneous and compact, with clear 

 flow-structure. The inclusions, when present, are always fragments 

 either of igneous rock similar to the ground-mass or akin thereto, 

 or (and) of the contact-rocks of the sills or dykes: wherever 

 inclusions occur, some at least are fragments of the contact- 

 rocks. The igneous inclusions show clear signs of having been 

 subject to surface-solution by the ground-mass : the shale-inclusions 

 have been softened, and in sonic cases greatly deformed, being 

 drawn out into mere wisps. Some of . the agglomeratic rocks are 

 certainly dykes and not sills, and as such cannot be interbedded 

 tuffs. The best and clearest example of conformable inter- 

 stratification is a sill which sends apophyses into the neighbouring 

 -hales, and these apophyses alter the shales at contact. Every 

 exposure at some place irregularly invades the contact-shales. 



On the whole of the evidence adduced above, and now summarized 

 incompletely, the 'dark igneous 1 rock obviously cannot be a tuff. 

 The even and parallel planes which bound it, and the presence of 

 apophyses, preclude the idea of an effusive lava. 



There remains the possibility that matrix and igneous fragments 

 are both derived from successive intrusions of practically the same 

 magma, along planes of continued weakness- the later intrusions 

 pressing farther forward, and taking up fragments of the earlier 

 in their course. In this manner it must have been that the 

 ' dark igneous ' rock got Its inclusions of micropegmatite, and in 

 this manner the Meldon aplite took up the large inclusions of the 

 •dark igneous' rock. If we extend these well-ascertained facts to 

 the period of the 'dark igneous' series itself, the interpolation 

 seems not only justifiable but unavoidable. 



The fact that the 'dark igneous" rocks are confined to a band, 

 even if that band be 2000 feet wide, is not overlooked: it affords 

 no valid argument against their wholly intrusive nature, since the 

 .Meldon aplite, undoubtedly an intrusive rock, is itself confined to 

 a band of approximately the same breadth, overlapping but not 

 conterminous. 



