176 GEOGNOSY. [Boox IT. 
The subjoined analyses show the composition of some varieties of 
clay ironstones.* 



Clay iron-ore Black Band Cleveland ore 
(Coal measures), (Carboniferous), (Lias), 
Yorkshire. Scotland. Yorkshire. 
Peroxide'of iron .. 2 %°.. 1. 649 (24 % 22°72) Cee 
Protoxide of iron. . . ... so'l4 . . 40°77 .) eee 
Protoxide of manganese aah qk Oe onedaae a= . 0°40 
Aluniina..10¢ Ors. a Omi eee — een 
Lime i 2) weet ew 22 ZETON 271. 20 00 ee 
Magnesia . .y)-) er ee he PARLT on ps 20 Toe 
POtaS Fo seiko) fue a ee Oe ca oe — 
Silica’. . tS Oe Se Oe Paar ee L010 
Carbonic acid 5 RP QO SOT Os - 126041) eee 
Phosphoric acid.;. 4..).« siya Oper — . «> BB 
Sulphurieacid = . . «9. « ) Wace 4) re 
Tronipyrites |. "<2! 2 te Es — — 
Water’. ..)). 3 8 Sas ee ee 3°48 
Organic matter...» <7. 4 jaqg 2a os 0°15 
93°7s 100: 00 100°61 
Percentage.of iron . . . . 2912 . . 3St°60 | leo mG 
§ vii. DETERMINATION OF ROCKS. 
Three methods of procedure are available in the examination and 
determination of rocks: 1st, the rough and ready but often sufficient 
appliances for examining macroscopic characters in the field or in- 
doors ; 2nd, microscopic investigation ; 3rd, chemical analysis. 
i. Macroscopic Examination in the field or 
indoors. 
The instruments indispensable for the investigation of rocks in the 
field are few in number and simple in character and application. 
The observer will be sufficiently accoutred if he carries with him a 
hammer of such form and weight as will enable him to break off 
clean sharp unweathered chips from the edges of rock-masses, a 
small lens, a pocket-knife of hard steel for determining the hardness 
of rocks and minerals, a magnet or a magnetized knife-blade, and a 
small pocket phial of dilute hydrochloric acid. 
Should the object be to form a collection of rocks, a hammer of at 
least three or four pounds in weight should be carried ; also one or two 
chisels and a small trimming hammer, weighing about + 1b., for reducing 
the specimens to shape. A convenient size of specimens is 4x3x1 
inches. ‘They should be as nearly as possible uniform in size, so as 
to be capable of orderly arrangement in the drawers or shelves of a 
ease or cabinet. Attention should be paid not only to obtain a 
thoroughly fresh fracture of a rock, but also a weathered surface 
wherever there is anything characteristic in the weathering. Every 
specimen should have affixed to it a label indicating as exactly as 
* See Percy’s Metallurgy, vol. ii. Bischof. Chem. und Phys. Geol., Supp. (1871) p. 65, 

a 
