326 J. ARTHUR PHILLIPS ON THE ROCKS OF 



tion of a coulee formed under conditions similar to those aifecting the 

 lava-flows from more modern volcanoes. 



By weathering, the calcite and other secondary minerals filling the 

 cavities become removed, leaving the rock in the form of a cavernous 

 mass of a rusty brown colour, produced by the peroxidation and hy- 

 dration of the iron present. 



On treating the pulverized rock with cold dilute acetic acid, small 

 quantities of ferrous oxide and alumina are dissolved along with the 

 calcareous or dolomitic filling of the vesicles. If, however, these be 

 regarded as forming part of the original igneous rock, and the com- 

 bining capacity of carbonic anhydride be satisfied from the dissolved 

 lime and magnesia, the results of an analysis made in duplicate may 

 be expressed as follows (sp. gr.=2-80) : — 



Proportion of carbonate to rock — 



Mean. 



Carbonates 25-73 



Rock 74-27 



100-00 

 The carbonates are composed of 



Carbonate of calcium 96-20 



„ magnesium 3*80 



100-00 



The percentage composition of this rock, deduction made of the 

 carbonates, is as follows : — 



I. II. 



Water* I h yg r&metric *..---•• '^ '93 



Water 1 combined 4*38 4-58 



Silica 48-66 48-37 



Phosphoric anhydride trace trace 



Alumina 23-27 22-92 



Ferrous oxide ' 13-12 13-03 



Ferric „ '77 1-01 



Manganous oxide trace trace 



Magnesia 3-20 3-18 



Potassa *57 *63 



Soda 5-17 h-22 



100-06 99-87 



When thin sections are examined under the microscope they are 

 seen to consist of a base, principally composed of elongated crystals, 



* Hygroscopic moisture of original rock added to weight of dried residue 

 operated on. In the case of this and of other rocks containing a large quantity 

 of ferrous oxide the combined water was collected in a chloride-of -calcium 

 tube. 



