342 A. R. Hunt — Devonian Bocks of South Devon. 



meet with both ordinary fine-grained diabases, and diabases which 

 have almost lost their original minerals by chemical processes. At 

 Wiuslade quarry, in the same block, we note an ordinary diabase, 

 and a chloritic rock whose augites are almost past detection. Near 

 the Start we find a metamorphosed volcanic rock, apparently a 

 plagioclase-pyroxene variety, which may well have been first altered 

 by chemical action, and subsequently sheared and metamorphosed 

 dynamically, with development of fibrous hornblende and zoisite.^ 



The evidence of the above rocks, from Torbay, and more especially 

 from Dartmouth, westward, seems to indicate that the passage is from 

 rocks affected by a high temperature and pressure, to rocks affected 

 by a higher temperature and pressure, the one below and the other 

 above the heat requisite for the ready solution of the quartzose, 

 scliorlaceous and felspathic components of the rocks involved ; 

 hydrothermal conditions being prevalent in each case. 



The Charactek of the Metamorphosis, 



Hydrothermal action would probably suffice to account for the 

 passage of the micaceous sandstones into quartzites, and with 

 pressure added, into quartz-schists ; but simple hydrothermal action 

 Avill not meet the case of the altered diabases. Even so far east as 

 Dartmouth we find an uncrushed diabase at Sandquay with its 

 original augite crystals nearly obliterated, and we know from the 

 experiments of M. Danbree that augite is not affected by plain 

 superheated water." 



The augite crystals of the South Devon diabases are often pierced 

 by laths of felspars : felspar in decomposing would tend to produce 

 silicate of aluminium — clay. According to Miller, "the inter- 

 mixture of lime magnesia and oxide of iron with clay . . . causes 

 it to be more readily attacked by acids." ^ In a decomposing crystal 

 of augite pierced by felspar, we have the exact conditions for ac- 

 celerating the action of acids, viz. the association of lime, magnesia, 

 and iron with clay. In ordinai-y cases water, charged with carbonic 

 acid by decomposing vegetation, percolating through the diabase, 

 might in the presence of heat go far towai'ds the production of 

 secondary felspar, chlorite, and hornblende ; but the complete meta- 

 morphosis undergone by the green schists seems to call for more 

 active agents in the form of strongly acidulated mineral waters. 



In the green schists of Bickerton and Eickham Sands, we have 

 granules of felspar,^ probably albite, embedded in a matrix consisting 

 chiefly of chlorite, and in the former occasional epidote and carbonates. 



In the diabases we have crystals of augite in a matrix of felspar, 

 probably a lime-soda felspar. In the Southern rock we have a 

 maguesian matrix, and in the Northern a non-magnesian matrix. 

 For the passage of the Northern rock into the Southern, as repre- 



' Appendix, flide 40. 



- Geologie Experimentalc, vol. i. p. 207. 



•* Elements of Chemistry, vol. ii. (third edition), p. 509. 



* In these rocks there is often a marked alineatiou of the felspar. 



