20 ME. G. W. LAMPLUGH ON SOME EFFECTS OF [Feb. I9OO,. 



into closed folds and are sheared in every part. It is true I found 

 indications that the material was further sheared after it was 

 brecciated, 1 but I regarded the two structures as practically 

 inseparable, and both seemed to have been produced under a 

 superincumbent weight sufficiently great to cause movement 

 analogous to fluxion throughout the mass. 



In the Manx Volcanic Series, however, the rocks have been 

 packed together and shattered in a well-defined localized belt, 

 without any perceptible change of intimate structure, with little or 

 no flattening of the fragments, and without any marked effect upon 

 the earlier Carboniferous strata outside this belt. We might say that 

 they have locally undergone ' breccia tion-wi thou t-cr us hing,' 

 in contradistinction from the ' breccia tion-with-crushing' of 

 the Slates. Moreover, the disturb auces seem to have been limited in 

 the horizontal as well as in the vertical plane, and to have been almost 

 confined to strata of a particular lithological type. It is probable 

 that the movement did not even extend down to the base of the 

 limestone, which is likely to be at a depth of not much more 

 than 200 feet below the Volcanic Series in this locality ; and the 

 apparent freedom with which the disturbed rocks have burst 

 upward indicates a comparatively superficial plane for the belt of 

 disturbance. 2 In studying the sections, one has indeed been led 

 to speculate upon the possibility of the effects having been produced 

 by the slipping outward of the margin of a steep submarine volcanic 

 cone of which we might see only the edge in these exposures, or' 

 by some other form of local movement accompanying the later 

 stages of the eruptive period. No such explanation, however, 

 seems sufficient, and I think that the phenomena are more pro- 

 bably part of the wider system of disturbances which has affected 

 the Lower Carboniferous rocks of the Isle of Man in common with 

 rocks of the same age in the east of Ireland on the one side, and 

 in the country south of the Lake District on the other. 



However that may be, it is clear that the factor of prime 

 importance in determining the extraordinary features of these 

 sections has been the varying degree of resistance to lateral 

 pressure offered by this heterogeneous sequence. If the whole 

 series had been composed of limestone, it is easy to believe that the 

 requisite degree of contraction would have been obtained by a 

 simple system of folding. But the rigid lava-flows and sills inter- 

 bedded among semi-coherent ash under no great weight of super- 

 incumbent material have proved incompressible, and when tilted 

 up by earth-creep have snapped into huge segments and ploughed 

 into the surrounding tuft's, which have been rearranged and 

 repacked around them. The unequal pressures thus brought to 

 bear upon the floor of limestone have rucked up that material into 



1 Quart, Journ. Geol. Soc. vol. li (1895) p. 585. 



2 It may be remarked that Prof. C. R. Van Hise, in discussing ' autoclastie ' 

 structures in general, states that brecciation is probably confined to a com- 

 paratively moderate depth in the earth's crust ; see his ' Principles of North 

 .American Pre -Cambrian Geology ' 16th Ann. Rep. U.S. Geol. Surv. pt. i (1896) 

 pp. 679-80. 



