24 British Association Beports. 



to investigate and report upon the igneous rocks of Staffordsliire, 

 and he now came forward merely to report progress — ^not having 

 yet been able to make a complete examination. After having 

 collected all the published information on the subject that was 

 attainable, he found that our chemical knowledge was confined 

 to some two or three analyses made at random. The two analyses 

 of Rowley Eag, made by Mr. Henry, and inserted in Mr. Beete 

 Juke's memoir, represented no doubt the exact composition of the 

 hand specimens sent to Mr. Henry, whose skill was unquestionable; 

 but he was satisfied that they did not represent the composition of 

 the rock masses in general, and would not be such as a geologist 

 would select to represent what he considered as a fair specimen 

 of what the rock mass really was. In these investigations it 

 was absolutely essential that the chemist, geologist, and miner- 

 alogist should go hand-in-hand. It was, therefore, necessary to 

 make a personal visit to each of the principal localities where these 

 rocks occurred, and to select specimens which would fairly repre- 

 sent the unaltered rock mass distant as far as possible from the 

 external surface, which naturally was invariably more or less 

 altered by the action of air, water, carbonic acid, etc., as also 

 from the intermixture and absorption of the surrounding strata with 

 which it came into contact. A series of specimens were submitted 

 to chemical examination, and at the same time thin sections had been 

 carefully prepared for the microscope, thus showing that the rocks 

 (which to the naked eye present nothing btit a dark indistinct sur- 

 face, as if composed wholly of one mineral) are resolvable into the 

 various minerals which actually entered into their composition. 



Their specific gravity was also determined. Eocks had been 

 analyzed from seven different localities, and microscopical examina- 

 tions made of about fifteen localities. As might be seen from 

 the map of the Ordnance Geological Survey, the bosses of igneous 

 rock which presented themselves at the surface were some thirteen 

 in number, the most extensive of them all being the Eowley 

 Hills, covering an area of probably about two and a-half square 

 miles ; after which the Wednesfield and Barrow Hills eruptions 

 came respectively next in extent, whilst the remainder were on 

 a much smaller scale. Besides, however, such rocks as were visible 

 on the surface, the extensive mining operations for coal had 

 disclosed numerous dykes cutting through the Coal-measures, and 

 frequently forming large masses, or more or less regular sheets 

 of rock imbedded in the strata. These were generally less compact 

 in appearance and of lighter colour than the larger masses, and were 

 known to the miners of the district as " green rock," presenting 

 some resemblance in external appearance to a greenstone, but in 

 reality quite distinct from that rock- A third variety of igneous 

 rock formed small dykes and veins, often very irregular and alter- 

 ing both the coal and the rocks in contact with it. It was known 

 to the miners as "white horse," and was occasionally called " white 

 trap," or feldspathic trap." All these rocks, however dissimilar in 



