British Mineralogy. 347 



Titanic acid .... 21-87 34'28 



Oxide of iron .... 41*93 65-72 



~^m 10000 



And if the iron and titanium are supposed to be present respec- 

 tively as titanic acid (TiO 2 ) and sesquioxide of iron (Fe 2 3 ), 

 according to the views of Rose and Berzelius, this composition 

 closely approximates to the formula Fe 2 O 3 TiO 2 , which, when 

 the equivalents of titanium and iron are respectively considered 

 to be 25 and 28, requires — 



Titanic acid (TiO 2 ) ... 41 33-89 



Sesquioxide of iron (Fe 2 O 3 ) _80 66-11 



121 100-00 



The minerals associated with this titanoferrite are seldom to be 

 distinguished by the naked eye, but are easily recognized by a 

 microscopic examination of thin sections of the rock itself*", and 

 are seen to be : — a triclinic soda-lime felspar, probably Labrado- 

 rite ; augite in brown crystals often finely developed ; and a small 

 quantity of a green mineral having the same crystalline form as 

 the augite, which may possibly be Delessite (a ferruginous chlo- 

 rite), but is more probably seladonite, a variety (hydrous) of 

 augite. In some specimens specks of iron pyrites are seen, and 

 occasionally minute long prisms which are hexagonal in section 

 and most likely are apatite, as by a chemical examination a small 

 amount of phosphoric acid was found in some specimens. In 

 some instances the rock contains small radiating crystallizations 

 of a zeolitic mineral, occasionally from one-eighth to a quarter of 

 an inch diameter, which, although not as yet examined, appears 

 to be pectolite. 



The geological age of the intrusion of these rocks is considered 

 by Mr. Beete Jukes to be contemporaneous with that of the coal- 

 measures, but is believed by the author to have been subsequent 

 to the deposition and consolidation of these strata. It is not 

 improbable, however, that it was previous to the formation of 

 most of the Permian series ; since at Marston Jabet in War- 

 wickshire, and at the Holland Arms in Anglesea, eruptive rocks 

 analogous to these are found traversing the carboniferous strata, 

 but overlain by Permian beds, which consequently must be of 

 later date. 



The occurrence of these insignificant, almost microscopic black 

 specks of titanoferrite in the substance of these rocks, should they 

 even happen to attract the passing attention of the field geologist 

 at all, would most probably be quite disregarded by him, in his 



* Vide an article by the author " On the Microscope in Geology," in 

 the Popular Science Review for October 1867, plate 17. fig- 5. 



2 A2 



