I. GEOLOGY. 695 



In this section crystals of magnesian mica are shoAvn lying in a felsitic 

 matrix. 



6. Diabase, x 25. Knowles Hill, Newton Bushel, Devonshire. 



This rock is mainly composed of an altered triclinic Felspar, Augite, Chlorite, 

 Serpentine Quartz, and Pyrites apparently altered in places into Limonite. A 

 little Apatite is also present. In the drawing the upper half of the field 

 represents the section by ordinary illumination, while the lower half shows 

 the remaining portion as seen by polarized light. 



7. Minette, x 150. Washfield, three miles N.W. of Tiverton, Devonshire. 

 A much decomposed Minette, full of small cavities, which cause it to 



resemble a scoriaceous lava, but when examined microscopically many of 

 these holes are seen to present definite hexahedral forms, from which no 

 doubt crystals of mica have been removed. The black granules and crystals 

 represent Magnetite in some, if not in all, instances. The matrix is felsitic. 



8. Microscopic drawings illustrative of structure in some of the eruptive 

 rocks of Somersetshire, Fig. 1, volcanic breccia x 25. Wrington Warren, 

 near Bristol. Section etched with acid and drawn as seen by reflected light. 

 The upper portion of the field represents part of a small fragment of fossili- 

 ferous limestone (showing crinoid remains). 



Fig. 2. Portions of crinoid stems from one of the above limestone frag- 

 ments, x 105. 



Fig. 3. Doleritic matrix in which the above limestone fragments are 

 imbedded, x 50. 



Fig. 4. Crystals of Magnetite in Felstone, x 150. Near Downhead, 

 Mendips. 



Fig. 5. Amygdaloid in 'Dolerite, x 25. Uphill Cutting, Great Western 

 Railway. 



Fig. 6. Amygdaloid of Calcspar in Dolerite, x 45. Uphill Cutting, Great 

 Western Railway. 



8a. Mechanical autotype of Plate 8, by Messrs. Vincent Brooks, Day, and 

 Son. 



9. Microscopic drawings illustrative of structure in some of the eruptive 

 rocks of Somersetshire. 



Fig. 1. Pseudomorph after Augite, from Dolerite, x 175. Uphill Cut- 

 ting, Great Western Railway. 



Fig. 2. Outline of an Augite crystal (natural size), given for comparison 

 with preceding figure. 



Fig. 3. Plagioclase from Doleritic matrix of Volcanic Breccia. Wrington 

 Warren, near Bristol. 



Fig. 4. Ditto, showing banding by polarised light, x 100. 



Fig. 5. Pseudomorph after Augite, from Dolerite, x 175. Uphill Cut- 

 ting, Great Western Railway. 



Figs. 6, 7, and 10. Calcareous bodies in Volcanic Breccia. Wrington 

 Warren, near Bristol. 



Fig. 8. Crystal of a Felspar in diversified Pitchstone Porphyry. Stoke 

 Lane, Mendips. 



Fig. 9. Calcspar Amygdaloids in Dolerite, x 50. Uphill Cutting, Great 

 Western Railway. 



Fig. 11. Crystal of Titaniferous Iron from Felstone, x 350. Near Down- 

 head, Mendips. 



Fig. 12. Fragment of Volcanic Breccia (natural size). Wrington Warren, 

 near- Bristol. 



The matrix is Doleritic, the imbedded fragments are Fossiliferous Lime- 

 stone, probably of Carboniferous age. 



Figs. 13 and 14. Magnetite crystals in Felstone, x 350. Near Downhead, 

 Mendips. 



9a. Mechanical Autotype of Plate 9, by Messrs. Vincent Brooks, Day, and 

 Son. 



