J. W. Jackson — Occurrence of Productus humerosus. 507 



black grains and examining them under the microscope the 

 fragments exhibit the deep brown colour in transmitted light and 

 the isotropic character of chromite. In several instances a green 

 serpentine is associated with the chromite. The other heavy- 

 minerals present are tourmaline, staurolite, biotite, zircon, r utile, 

 and andalusite, none of which call for special comment. 



A second sample gave 0"28 per cent sandy residue, but no 

 chromite was seen in it. A thin section of the stone showed as 

 many as seven fairly large quartz grains in an area of about half 

 a square inch. 



Detrital chromite does not seem to have been recorded in any 

 Cretaceous or older deposits in England. From this fact, 

 as well as from the coarseness of the grains and the patchy nature 

 of the occurrence, we may conclude that the chromite was derived 

 directly from some area of ultra-basic rocks, possibly the Lizard, 

 possibly an area now submerged beneath the English Channel ; and 

 the occurrence is of interest as showing that serpentines as well 

 as granites were being eroded in Turonian times. 



IV. — On the Occurrence of Productus humemosus ( = subLjEvis) 

 in Dove Dale ; and its Yalue as a Zone-fossil. 



By J. Wilfrid Jackson, F.G.S., Manchester Museum. 



DURING a recent examination of the Carboniferous Limestone in 

 the Dove Dale district I have met with an abundance of 

 Productus humerosus. 1 This species is a common fossil in the cliffs 

 •extending for a distance of quite 2 miles from the top end of Nabs 

 Dale to beyond Tissington Spires. It is most prolific at and near 

 Reynard's Cave, where the shells occur in clusters, often cupped 

 into each other. 



The beds containing this fossil are apparently not far below the 

 top of the limestone, probably not more than some 500 feet. 



The limestone is dark-grey in colour, with very obscure bedding. 

 It breaks up irregularly into angular fragments, and contains some 

 crinoid debris in places. Here and there occurs an organism of the 

 nature of a Stromatoporoid, which invests the various fossils. 

 There is an absence of chert, the limestone being very pure. 



In addition to P. humerosus the beds contain Amplexus coralloides 

 (very common), and several interesting Brachiopods, Gastropods, and 

 Lamellibranchs ; also Trilobites and Fenestellids, the fossils occurring 

 in nests in certain places. 



The specimens of P. humerosus include several varietal forms, 

 which may or may not deserve distinctive names. Pending an 

 exhaustive study of the material from Dove Dale and Caldon Low 

 I here use the specific name humerosus to cover all such variations. 

 Four such forms may be distinguished, but these are linked together 

 by intermediates : — 



1. P. aff. suhlcBvis, de Eon., almost smooth, narrow and highly 

 convex, with no trace of median sinus ; flattened down median area 

 from the beak to the anterior margin. 



1 First recorded in Geol. Mag., July, 1919, p. 335. 



