142 



Robert A. Kehle: 



Fourthly — Sedimentary rocks. 



(a) Palaeozoic sandstones shales and slates. 



(b) Jurassic sandstones and sliales. 



(c) Tertiary sediments. 



(d) Decomposed igneous rocks, particularly the lavas 



of the Older Basalt. 

 It will be recoirnised that the structure and physical characteris- 

 tics of these several rocks modify their powers of resistance. The 

 metamorphic locks, placed above as the most resistant, merge into 

 one of the least resistant at a variable distance from the actual 

 -contact; they retain more or less of their tectonic structure, and 

 in this respect are subject to the same erosional factors as their 

 unaltered lepi-esentatives. Igneous rocks decrease in resistance 

 according to Avhether they are vitreous, hypocrystalline or holo- 

 '«rystalline. The marked difference in resistance between those 

 secondly and thirdly tabulated seems to be due to this rather than 

 to any decided dissimilarity in chemical composition. The Palaeo- 

 .2'oic sediments as a whole offer little resistance compared to the 

 igneous rocks, but their sandstone members are sometimes more 

 resistant than some igneous rocks. 



Short cycles of erosion during Volcanic activity. 



Short cycles of erosion were initiated at the beginning of the 

 comparatively brief periods of quiescence between the lava flows 

 ■comprising the Older Basalt, and lateral streams commenced to 

 -cut back from the changes of gradient at the edge of the confined 

 lava fields. Erosion had not proceeded to any extent before 

 another lava flow filled in the young valley, and a new cycle com- 

 menced at the edge of the last flow. Infilling and erosion thus 

 proceeded hand in hand until a cycle of erosion — the Intermediate 

 — proceeded uninterrupted. The inferences arising from this 

 sequence of events is both interesting and important. A section 

 '(Fig. 5) will better illustrate the possibilities arising from it. 





Fig. 5. 



Erosion succeeding? three consecutive lava flows with a compuratively 

 short space of lime between them. Hypothetic;\.l Section. 



