88 



ME. W. A. R1CHARDSOX OK THE PETROLOGY [vol. lxxiX, 



the fauna of which country is perhaps better illustrated than that of 

 other regions, only one doubtful form (Ammonites falcaries olifex 

 Quenstedt [/>fl?*s]) represents the abundant Arnioceras fauna of 

 the period. Faunas so poorly preserved as that of the tuberculatus- 

 shales of Wurtemberg have unfortunately not received much 

 attention in the past ; but, in order to arrive at a more satisfactory 

 correlation of the Lower Lias deposits in the various areas, it is 

 necessary to collect, with the care and diligence that Dr. Lang has 

 bestowed on the Dorset coast, what (at first) looks like unpromising 

 material. 



Part III. Petrology. (By W. A. E.) 



(A) Laminated Shales. 



Shales, which are minutely laminated at the outcrop, but appear 

 massive when traced inwards, have been described in Part I (p. 53). 

 The change is accompanied by progressive bleaching towards the 

 weathered surface, and by the deposition of minute, platy crystals 

 of selenite between the laminae. This bleaching suggests that the 

 development of lamination has been accompanied by a loss of 

 certain constituents of the shales, such as carbon (or hydrocarbons), 

 and also (as indicated hj the appearance of gypsum) of iron- 

 sulphide. Thus the weathering of the shales has produced a 

 diminution of volume, and the resulting contraction, by separating 

 the individual laminae, has revealed the minute internal structure 

 of the shales. 



It may be noted that similar development of lamination may be 

 seen, for example, in the Phytic Black Shales at Beacon Hill, 

 Newark. ^ 



(B) The Occurrence of Barvtes. 



Fig. 3. — Disc of barytes 

 from bed 71 e, Charmouth. 



C fois. 



?rrr — 



1> werftons I Pjan. 



In bed 71 e, consisting of well- 

 stratified blue shale, Dr. Lang has 

 found thin biconvex discs of a 

 mineral determined by Mr. W. 

 Campbell Smith as barytes. These 

 discs are irregularly distributed 

 along bedding-surfaces, from which 

 they are readily detached, leaving 

 behind a perfect impression of their 

 shape and markings. The general 

 appearance of these barytes discs 

 can be gathered from fig. 3. The 

 largest specimen examined had a 

 mean diameter of 15 mm., a maxi- 

 mum thickness of 2 mm., and the 

 specific gravity = 4"58. Both the 

 upper and lower surfaces of the 

 discs are marked by radial furrows, 



