580 MR. G. W. LAMP1TTGH ON THE [NOV. 1895,- 



that my impression, based on the evident repetition of some of the 

 beds by folding, is that there is approximately only one horizon 

 represented in the various conglomeratic bands, but that this horizon 

 has been somewhat modified by a considerable variation in the thick- 

 ness of the strata involved in the disruptive process. Nevertheless 

 the lithological character of the Manx slates as a whole calls for 

 great caution in dealing with this matter. Over large areas, both 

 on the coast and in the interior, we find beds intermediate in 

 texture between grits and shales, composed usually of alternating 

 laminae of impure argillaceous and fine-grained arenaceous material, 

 often in vividly contrasting pale grey and blue-black tints ; and 

 among these rocks thin bands of grit or quartzite frequently recur, 

 both isolated and in groups. No sharp line can be drawn between 

 such rocks and the true grits, on the one hand, and the dark-blue 

 smooth argillaceous slates on the other. Even beyond the limits 

 of these intermediate beds and well within the region of the blue 

 clay-slates themselves, we shall find that layers of dark grit a few 

 inches in thickness are not infrequent, showing that the source of 

 the arenaceous material was still open. Under such conditions we 

 cannot but expect to meet with some interchangeability of litho- 

 logical type, and the development of sandy rocks in one place and 

 of shales in another on the same horizon. 



In spite of this, however, we find that the central ridge of the 

 Island, including all the highest summits, is from end to end built up 

 almost entirely of one type of rock, a smooth, dark, much sheared 

 and imperfectly cleaved clay-slate, which from its occurrence 

 both in North Barrule and South Barrule — hills 16 miles apart, at 

 the opposite ends of the range — I propose to name 'The Barrule 

 Slates.' We find also that the flags and grits are distributed 

 chiefly on either flank of these Barrule Slates, with an ill-defined 

 intervening zone of banded rocks of intermediate texture, and that 

 the flaggy series, or its associated banded slate, is reached in most 

 of the deeper valleys. And this arrangement leads me to think 

 that, taken as a whole and in a broad sense, the gritty rocks of the 

 Island do form a definite stratigraphical horizon, and that the 

 variable beds mark the vertical passage from this sandy set into 

 the slaty series. It is evidently in these passage-beds that the 

 brecciation has chiefly taken place ; and where the crush-conglom- 

 erates are very thick, as in Sulby Glen, it appears probable that the 

 intermediate series has been almost entirely disrupted and redis- 

 tributed as a shear-breccia between the Barrule Slates and the 

 fundamental grits. 



(1) Method of Formation. 



We return to surer ground in discussing the method, and the age, 

 of the brecciation as revealed in the intimate structure of the crush- 

 conglomerates. 



Not even the experimentalist could devise a better material for 

 rendering evident the effects of earth-movements than the colour- 



