218 W. M. Hutchings — Ash-slafes of the Ldlce-District. 



and the disturbed area may be attributed to variations in the amount 

 of slip tliroughout the seismic focus. (1) If the amount of slip be 

 everywhere very small, the sound-focus may occupy the whole of 

 the slip-area, and thus sound may be the only phenomenon per- 

 ceptible at the surface to the unaided senses. This seems to have 

 been frequently the case amongst the series of small slips which 

 jDroduced the numerous slight shocks at Comrie, Pignerol, and else- 

 where. (2) If the sound-ibcus occupy nearly the whole of the 

 slip-area, the amount of slip in the rest of it being small, but still 

 great enough to produce a slight shock, then the sound-area and the 

 disturbed area might be approximately co-extensive, or the sound- 

 ai'ea might in places entirely overlap the disturbed area. (3) But 

 very frequently, especially in the more pronounced seismic areas, 

 the maximum amount of slip within the seismic focus will be so 

 gi'eat that the disturbed area will be large compared with the 

 sound-area, and, in severe earthquakes, will extend far beyond it. 

 (4) Lastly, the slip might take place suddenly, and its amount be 

 so great, that the sound-focus might be confined to the lateral 

 margins of the slip-area. The slip would then extend up to the 

 surface of the earth, and, if great enough, might be traceable there 

 as a difference of elevation on the two sides of the fault-line ; the 

 sound-area would consist of two detached portions at some distance 

 from the region of maximum disturbance, and the sounds con- 

 sequently might escape observation and record. 



But while earthquakes of such extreme intensity are very unusual, 

 very slight slips must frequently take place ; so that earthquake- 

 sounds without an accompanying shock should be of far more 

 common occurrence than earthquakes without attendant sounds. 



V. — Notes on the Ash-slates and other Kocks of the Lake 



District. 



By W. Maynard Hutchings, Esq. 



{^Concluded from page 161.) 



TAKING the other and coarser constituents of such slates as are 

 not wholljr made up of the fine "base," — the constituents which 

 may be spoken of as " porphyritic," — the lapiJli vary very greatly in 

 number and distinctness. In a large part of the roofing-slates they 

 are either no longer discernible at all or are so exceedingly faded 

 and blurred as to be just barely recognizable, often as patches 

 altered to chlorite, or chlorite and calcite, in which the felspar-laths 

 of the original andesitic ground-mass may still be seen comparatively 

 little altered. In cases where there is reason to suppose that the 

 lapilli were largely of more basic nature, this almost complete 

 alteration of them is observed, as might naturally be expected. 



In other cases the crushing and rolling-out of the rock has sufficed 

 to obliterate all traces of original fragments of whatever sort. On 

 the other hand, there are many slates in which lapilli, in great 

 abundance, are still so perfectly preserved as to exceed in freshness 



