F. P. Mennell — The Northern Margin of Dartmoor. 417 



II. The Taw Section. 

 Turning now to the River Taw, which crosses the granite contact 

 about a mile east of the Okement, we have a very good section of the 

 lower beds, though scarcely anything is to be seen of the outermost 

 members of the series we have been discussing. The river ceases to 

 flow at right angles to the strike as soon as it gets into the lower 

 limestone, and this fact has also to be taken into account. There 

 are some interesting minor differences between the Taw and Okement 

 sections, but on the whole they correspond very well. Taking what 

 can be seen above the road west of Sticklepath, as well as what the 

 river shows, we have this succession, in which the beds are numbered, 

 where they can be identified, in the same way as along the 

 Okement: — 



8. silicated limestone. 



x. epidiorite. 



7. cordierite hornfels. 



x. epidiorite. 



6 (?). chiastolitic and spotted hornfels. 



x. porphyritic epidiorite. 



5. banded hornfels. 



4 (?). radiolarian rocks. 



3. tuff. 



2. silicated limestone. 



1. andalusite hornfels. 



Some of the beds worst seen along the Okement are extremely well 

 exposed along the banks of the Taw. The lowest member of the 

 series is, however, scarcely recognizable, apart from the artificial 

 exposures at Birchy Lake, and in stray fragments. The limestone 

 is splendidly exposed along the right bank of the stream and shows 

 a very varied mineralogical composition ; specimens containing coarsely 

 crystalline garnet, axinite, epidote, augite, hornblende, etc., being 

 readily obtained. There are in it some small mine workings (for 

 copper) now abandoned. The tuff is well seen on both sides of the 

 Taw, though thrusts are no doubt responsible for the fact that it is 

 absent, for a short distance, from the high ground between Belstone 

 and the river. Its total thickness seems to be about 60 feet. It 

 varies in grain from a fine ash to a coarse agglomerate, and, especially 

 at the top, there are numerous alternations of sedimentary material 

 with coarse ash bands a few inches thick. It is therefore clearly the 

 result of a lengthy period of volcanic activity and not of a single 

 paroxysmal eruption. 1 The rock fragments are of acid and inter- 

 mediate types, to the complete exclusion of basalts, as far as I have 

 examined them under the microscope, and are often highly 

 amygdaloidal. They include some very interesting rhyolites and 

 trachytes, surprisingly little affected by thermal alteration except for 



1 Mr. E. H. Worth has, since these notes were written, put forward the theory 

 that the tuff is of an intrusive nature. I am not aware of any evidence in 

 favour of that view unless the disconnected nature of the outcrops is considered 

 as such. The facts noted here are entirely incompatible with the rock being 

 anything but a true ash bed accumulated under the sea. 



DECADE VI. — VOL. VI. — NO. IX. 27 



