172 ME. JUKES-itROWNE AND PROF. HARRISON 



earths ; this locality is Mount Hillaby, where they are overlain by 

 about 25 feet of grey mudstones or felsjjathic earths, which are 

 mainly composed of fine volcanic dust. 



§ 2. The Minute Strtjcture of the Eocks. 



Samples of the principal varieties of the Oceanic Deposits of 

 Barbados were sent to Dr. John Murray, F.R.S.E., and to Mr. W. 

 Hill, F.G.S., while samples of those which appeared to contain much 

 inorganic mineral matter were sent to Prof. Bonuey, F.E.S. ; but, 

 his time being fully occupied, they were handed to Miss C. A. Eaisin, 

 B.Sc, who kindly undertook to examine them. 



Dr. Murray wrote to us in September 1889 that he had examined 

 the specimens we sent him, and that he believes "they are all 

 deposits formed in deep water. The red earths are very like some 

 of our red clays, and the deposits with radiolaria resemble our 

 radiolarian oozes. In the same way the calcareous deposits resemble 

 our Globigerina-oo7.es ; " and in a later letter he says " I think the 

 specimens could be identified as oceanic clays without knowing their 

 association with marls, but it would be difficult to fix a depth." 



This is brief but explicit testimony to the close correspondence 

 between the Barbadian deposits and the modern oceanic oozes.^ 



To Mr. Hill and Miss Eaisin we are indebted for full reports on 

 the specimens submitted to them. The information contained in 

 these reports is now presented in a combined and condensed form 

 for the convenience of the reader, but the portions furnished by 

 each writer are carefully indicated. 



Mr. Hill found that his specimens could be grouped under the 

 following heads: (1) Unmodified earths and marls; (2) Silicified 

 earths ; (3) Calcified earths or calcific limestones. The rest fall 

 under the heads of (4) Pumiceous deposits ; (5) Grey mudstones ; 

 (6) Fine clays or argillaceous earths. 



(1) Unmodified Sarths and Marls. — Mr. Hill reports as 

 follows : — 



These earthy rocks may be divided into calcareous, calcareo- 

 siliceous, siliceo-calcareous, and siliceous, according as the cal- 

 careous or the siliceous constituents predominate. There are marked 

 differences between the ])urely calcareous and the purely siliceous 

 rocks. Most of the former are like our chalks, dull white or 

 greyish pulverulent limestones, readily soiling fingers and clothes 

 with white dust. The siliceous rocks are lighter in weight, and 

 often emit a tinkling sound like coal-cinders when handled, while 

 the finer particles do not readily detach themselves. 



The following is a list of the specimens examined. The localitj' 

 and position of each are given, with a brief account of the leading 

 characteristics of each group as seen in thin sections under the 

 microscope with a |-inch objective. 



^ A remark to the same eifect appears on p. xxix of the E-eport on the Deep- 

 sea Deposits of the ' Challenger' Expedition, 1891. 



