A. Strahan — Chloritic Marl, Dorset. 319 



chlorite, and cooheite ; ' whilst topaz changes to steatite, mica, and 

 Tcaolin. 



In my Meldon specimens the green tint seen in some of the 

 tourmaline is probably due to the birth of ultra- microscopic particles 

 of chlorite in the originally colourless crystals. The topaz, on the 

 other hand, sometimes exhibits partial kaolinization, and when it 

 does so it is clear that the mineral is topaz and not tourmaline. 



On the whole, I think it probable that the somewhat partial and 

 patchy colour seen in the Meldon tourmaline is due to the post- 

 genital alteration of an originally colourless variety of this mineral. 

 I do not think the faint colour now visible is due to the bleaching 

 of an originally dark-coloured tourmaline. An operation of this 

 kind would imply the bleaching of the whole of the white granite 

 itself, which I do not think probable. Moreover, it would involve 

 chemical action to an extent that must have left very powerful 

 marks on the felspars and other susceptible minerals contained in 

 the rock, that could not remain unobserved. 



IV. — An Abnormal Section of Chloritic Marl at Mupe Bay, 



Dorset. 



By A. Strahan, M.A., F.G.S. 



IN " The Geology of the Isle of Purbeck and Weymouth" (p. 152) 

 I referred to some green sand which occurs next below the 

 Chalk in Mupe Bay, Dorset, as somewhat resembling Chloritic 

 Marl, but as being too thick for that bed. As it was followed by 

 the Gault, I concluded that it belonged to the Upper Greensand. 

 The section was subsequently visited by Mr. W. Hill, who collected 

 from the sand in question Holaster suhglohosus, var, alius, and 

 Echinoconus castanea among other fossils, and inferred that it was 

 Chloritic Marl. He considered that though it passed up insensibly 

 into the Chalk it was faulted against the Gault, but that there had 

 been also considerable contemporaneous erosion of the Upper Green- 

 sand. In April of this year I revisited the section in company with 

 Mr. Hill, and was fully satisfied as to the correctness of his views. 

 The following account has been drawn up from our observations : — 



The Lower Chalk becomes extremely impure in its lower part, 

 and contains much glauconite ; it thus gi'aduates insensibly down- 

 wards into a gritty glauconitic sand. The sand contains a few 

 phosphatic casts, more or less worn or corroded, scattered throughout 

 it, but has a well-marked nodule-bed crowded with these casts at 

 its base ; other fossils with the shell preserved and not filled in with 

 phosphate occur also throughout the whole bed. 



Below the sand comes a sandy glauconitic clay, forming part of 

 the Gault, but the fact that the separation is sharp, and that the 

 chert-beds of the Upper Greensand and the passage-beds down 

 into the Gault are absent, proves that the two are faulted together, 

 although the fault-plane is parallel to the highly inclined bedding. 



1 See Dana's textbook, last edition. 



