Physical Structure and older stratified Deposits of Devonshire. 659 



altered and mineralized. Some are yellow, earthy, and pyritous ; others have become entirely 

 earthy, are of a bright red colour, and, in their strike through the country, may be tracked 

 through the fields by the red colour of the soil. Others, again, are partially indurated ; and we 

 may here and there extract from them hard, laminated bands of quartz and chlorite ; and among 

 them are irregular masses of earthy, pyritous, decomposing chlorite. The beds, however, soon 

 acquire a more usual mineral structure ; undulate towards the south ; at Bolt Tail become nearly 

 vertical ; and afterwards pass, in a highly-inclined position, and after repeated undulations, to 

 the extreme southern headlands. The prevailing dip is, however, to the north ; and the strike is 

 nearly true east and west. 



In mineral structure, this group is entirely different from anything we have described in the 

 sections either of North or South Devon. It is highly crystalline, and essentially composed of 

 quartz, mica, and chlorite ; the three minerals being frequently found together. Near its northern 

 limit are some fine-grained, well-bedded rocks of blended quartz and chlorite, sometimes used as 

 a building stone. More frequently it rises in large flaky masses, coated beautifully with mica 

 and chlorite, and is used as a coarse flag- stone and coping-stone. In other places, from the oc- 

 currence of joints, the blocks are obtained in rude prisms, provincially called pencils, not unusu- 

 ally coated with red oxide of iron. The masses have both small and great contortions, so that it 

 is difficult to find beds with smooth plane partings. Towards the south, however, (e. g, near 

 Prawle Point,) we obtained beautiful and finely-laminated chlorite slate ; and there are similar 

 specimens of mica slate. On the whole, mica seems to abound more towards the southern, and 

 chlorite towards the northern, extremity of the group ; but the different varieties are so con- 

 stantly mixed together, that it is impossible to divide the crystalline series into two formations. 

 Felspar does not appear to exist as an essential constituent, nor did we see a single block to 

 which we should give the name of gneiss. Pyrites is found in many of the beds ; and oxide of 

 iron not merely coats the joints, in some cases producing what the workmen call iron pencil, but 

 the mineral exists in regular veins traversing the micaceous slate. The finest example is at Sea- 

 combe Sands, (near the mouth of the rivulet that descends from West Prawle,) where there is a 

 vein of iron ore about four feet wide, and another smaller vein near it. It strikes nearly east and 

 west, and underlies north, at an angle of 50°. Had it been more nearly vertical, it might, perhaps, 

 have been worked to profit. On the east side of Gunnor Head, near Prawle Point, is a similar 

 vein striking north and south, and underlying west at a great angle. Works have been opened in 

 tliis vein, but are deserted. There is also an iron vein at Stenhall, on the east side of Bolt Head, 

 and another about two miles further west. 



This beautiful but anomalous group is probably of metamorphic structure, and had entirely 

 escaped notice before the commencement of the elaborate county survey of Mr. De la Beche. 



4. — Section from Berry Head to Start Point. 

 (PI. LI. fig-. 6.) 



It only remains for us to describe in a few words the section connecting the Torbay limestone with 

 the crystallized slates of Start Point. We have before stated that the limestone of Berry Head 

 is thrown into an arch which brings up the inferior slates in the cliff of Mudstone Sands (p. 652) ; and 

 the same arch is continued in the calcareous system towards Galmpton. The earthy slates of Mud- 

 stone Sands become more and more calcareous and fossiliferous in the ascending section, and so 

 gradually pass into the limestone. In the same manner, on the north side of Galmpton Creek we 

 VOL. V. SECOND SERIES. 4 Q 



