Vol. 50.] IGNEOUS OBIGIN ON DAET.MOOR. 341 



The above specimens have a dark-grey matrix with blebby- 

 looking crystals of felspar embedded in it. Several specimens also 

 show rounded grains of quartz on their surface ; and all exhibit, 

 here and there, well-formed but rectangular crystals of felspar 

 which give these rocks the appearance of lavas. On the other hand, 

 many of these rocks when examined in the field contain numerous 

 undoubted fragments of slaty, and of volcanic rocks ; and the observer 

 is puzzled to know whether he has before him highly metamorphosed 

 tuffs, or lavas crowded with included fragments. 



The difficxilty here presented is not at once removed by an 

 appeal to the microscope, and I have never examined any rocks 

 regarding which I felt so much difficulty in finally making up my 

 mind. That these beds contain a vast number of fragments of 

 various kinds of lavas, and of slaty rocks, cannot be questioned. 

 But the cementing matrix, which encloses these undoubted frag- 

 ments, has so completely lost all trace of its original fragmentary 

 origin ; it so closely resembles the base of some quartz-porphyries 

 and some rhyolites, and has so entirely lost all trace of the agencies 

 by which the change was effected, that I was for long in doubt 

 as to whether I was dealing with a metamorphosed tuff, or whether 

 some, if not all, of the beds, were lavas that had caught up the 

 contents of an ash-bed in the course of their flow, or had been 

 profusely peppered with fine ash on their road from the crater to 

 their final resting-place. The conclusion at which I have finally 

 arrived is that the majority of the beds are metamorphosed tuffs, 

 but that in two or three cases the rocks are really igneous flows 

 that have in their passage through, or over, ash-beds caught up 

 numerous fragments of ejected volcanic and sedimentary material. 

 I propose to postpone further remarks on the subject until I 

 have described these, and some other specimens, from another 

 locality. I shall make my description as brief as possible and limit 

 myself to salient points. 



No. 6. This contains fragments of several kinds of lavas. The 

 rock has been considerably altered and contains much secondary 

 micaceous matter. 



No. 7. This contains numerous fragments of lavas and of altered 

 sedimentary rocks. One of the former class is a very dark glassy 

 lava, full of the dust of magnetite, and contains numerous small 

 oval vesicles and some microlites of felspar. Fragments of this rock 

 are very commonly met with in these tuffs. 



The interstitial matter in which the fragments are embedded is 

 like the micro-granular base of some rhyolites and porphyries ; and 

 it eats into, corrodes, and invades some of the fragments, some 

 portions of which appear to have floated off into the matrix and 

 to have become more or less completely detached from their parent 

 fragments. It is difficult to say offhand whether this corroding base 

 represents an intrusion into a tuff, or whether the tuff was partially 

 remelted after it was laid down. 



No. 8. This rock is composed of numerous fragments of the dark, 

 vesicular lava above described. The vesicles are stopped with a 



