ANALYSES OF SOME CAMBRIAN ROCKS. 377 



under the fossil have a somewhat nodular appearance. The entire 

 fragment is bulged out by an interior lenticular thickening, which 

 consists largely of calcic carbonate, whereas the layer in immediate 

 proximity to the fossil does not effervesce much with cold acid. 

 Hence this small fragment consists of two very different kinds of 

 rock. 



Small crystals of pyrites may be observed. The specific gravity 

 of the fragment is 2*84. 



The following analysis represents chiefly the composition of that 

 portion which adjoins the fossil impression and which does not 

 effervesce strongly in cold acid; it is not, however, altogether without 

 some of the interior and more calcareous portion. 



per cent. 



Silica, insoluble silicate, and pyrites 62-47 



Oxides of iron (calculated as ferrous oxide) . . 12*37 



Alumina 7.78 



Lime '. . . A 3-27 



Magnesia 2-25 



Phosphoric anhydride .... 1*62 



Difference, chiefly carbonic acid * 10*24 



100-00 



It is worthy of remark that there is a curious reddish brown 

 extractive matter which pervaded this analysis, and that a very 

 similar substance also pervaded the analysis of the lobster- shell, 

 hereafter to be noticed. 



No. II. Specimen in contact with trap. A dull black slaty rock, 

 joints lined with ferric hydrate, no pyrites visible. The impression 

 upon it of a Trilobite is distinct ; but there are no indications along 

 the costse of the decomposition of pyrites as in No. I. Neither is 

 there any lenticular thickening. The fragment is homogeneous. No 

 effervescence with cold acids ; some soluble silica. Specific gravity 

 2-69. 



per cent. 

 Silica and insoluble silicate (hardly any pyrites) 81*44 



Oxides of iron (calc. as ferrous oxide) 5-56 



Alumina 4*26 



Lime 0*44 



Magnesia 1*92 



Phosphoric anhydride f 0*11 



Difference, chiefly water J 6*27 



100*00 



There is none of the reddish brown extractive matter noticed in 

 the analysis of No. I. ; but the insoluble residue before ignition is 



* Containing, besides, water, organic matter, a very little alkali, and traces of 

 sulphuric acid. 



t The rock, when fused, exhibits no material increase of phosphoric acid. 

 I With some organic matter, carbonic acid, and alkali. 



