570 Correspondence — Messrs. Macnair and Reid. 



through some of the thick limestone bands of the district. Large 

 masses of vertical and crumpled limestone beds, as well as quartzite, 

 have been caught up in the agglomerate, together with abundant 

 blocks of grit, fragments of shale, and pieces of a pale felsite" 1 (five 

 varieties again). Of the vent at Porth Cenal my own description is : 

 "The most remarkable feature is the occurrence of great quartz 

 lumps, which are of all sizes and shapes, ami lie promiscuously in 

 an agglomerate of slates, grit, and dust," and, " in the headland of 

 Pen-y-parc we get another agglomerate of quartz lumps and ash " ; 

 and, after observing that these rocks (the limestone not being 

 mentioned as being merely torn from the sides of the vent) are 

 not those of the immediate neighbourhood, I added : " We cannot 

 here refer [these agglomerates] to the action of a crush-fault," 2 a 

 remark which shows that I was not unaware of this alternative. 

 These descriptions, with the interchange of " felsite " and " ash," 

 are fairly consistent, but Sir A. Geikie's new description is: "The 

 huge blocks of limestone, there to be seen isolated among frag- 

 mentary grits and slates, are referable to the disruption of some 

 of the limestone bands which occur abundantly in the neighbour- 

 hood " (quite so). " A gradation may be traced from the slates 

 and grits outside the areas of more severe dislocation into the 

 intensely crushed and sheared ' agglomerate.' " 3 Where is here the 

 quartzite, and the felsite, and the drilling? 



The fact is that one main reason, amongst others, in my mind 

 at least, for calling these Anglesey masses agglomerates, was the 

 occurrence in them of a variety of rocks not like those of the imme- 

 diate neighbourhood ; while the main reason for believing the Mans 

 rocks to be " crush-conglomerates " is that the rocks in them are 

 of those kinds only which occur on either side of the area of 

 crushing. The phenomena in the one case are, therefore, not the 

 same as those in the other. J. F. Blake. 



November 4, 1896. 



THE OLD EED SANDSTONE OF SCOTLAND. 

 Sir, — Regarding the statement of Professor Davis in last month's 

 issue of this Magazine, it gives us great pleasure to receive his 

 explanation that the terms Devonian and Old Red Sandstone had 

 been used synonymously in his paper, and not as representing two 

 distinctly different conditions of deposit. But he must be well 

 aware that this loose application of these two terms has long fallen 

 into disuse in British geology, and that they now stand for two 

 different types of deposit. Hence it was quite natural for anyone 

 reading his footnote to suppose that his "Devonian erosion" refers 

 to his notice of Sir A. Geikie's plain of marine denudation given in 

 the body of the paper; while his reference to the Old Red Sand- 

 stone stands for something quite distinct. Further, as he says 

 himself, having been principally indebted to "English writings" 

 for his knowledge of the structure of this country, we thought it 



1 Q.J.G.S., vol. xlvii, p. 134. 



2 Ibid., vol. xliv, pp. 517-8. 



3 Geul. Mag., Dec. IV, Vol. IN, p. 4S2. 



