96 GEOLOGICAL NQTES* 



ON THE OCCURRENCE OF PRE-CAMBRIAN 



SHALE IN GUERNSEY. 

 By George Horace Plymen, M.Sc, Ph.D., F.G.S. 



It was noted by the Rev. C. Noury 4 in 1886 that sedi- 

 mentary rocks were found in a small coastal area to the 

 south of Fort Pezerie, on the south-western extremity of 

 Guernsey. Professor T. G. Bonney 5 observed the same area 

 independently in September, 19 10, after describing it in field 

 notes in 1888 as a close- jointed and rather schistose diabase. 

 The Rev. E. Hill 6 in 1884 had also regarded the rock in this 

 area as constituting a dyke-formed diabase. 

 E. W. 



Fig. 1. — Pre-Cambrian Shale, Torteval, Guernsey. 



Professor Bonney, on submitting slices of the rock to 

 microscopical examination, defined it as a grit. He recog- 

 nized sub-angular and fairly rounded quartz, felspar with 

 (in cases) signs of pliagioclase, and dark brown, barely 

 translucent, iron oxide in grains. The matrix was observed 

 to be a mozaic of quartz and reconstituted felspar, with films 

 of mica, varying in colour from green to colourless. 



Although regarded as pre-Cambrian, this patch of sedi- 

 mentary rock has not a close similarity to the pre-Cambrian 

 shales of Jersey; but its occurrence in an area of intense 

 crushing and faulting, probably with accompanying meta- 

 morphism, gives reason for apparent dissimilarity. It may 

 well represent the quartzose * ' grey wacke ' ' beds, which 

 alternate with more typical mudstones, familiar to observers 

 of the Jersey sedimentaries and of the "Phyllades de St. 

 L6" in Normandy. 



While examining this small area in the spring of 1920, 

 with a view to correlation with the Jersey sedimentary beds, 

 the present writer discovered another outcrop near by, which 

 had hitherto escaped the notice of earlier investigators. It 

 was re-examined in April of this year. 



4 Geologie de Jersey, par R. P. Ch. Noury, S.J. 1886. 



5 Quart. Journ. Geol. Soc, vol. lxviii., 191 2, p. 47. 



6 Quart. Journ. Geol. Soc, vol. xl. ? 1884, p. 417. 



