398 MR. B. THOMPSON ON LANDSCAPE MAEBLE. [Aug. 1 894, 



they seem to differ from the matrix only in being finer grained. In 

 all cases the outer boundary of the dark markings seems to be de- 

 cidedly darker than the central portion. 



12. Other points brought out by the microscope are these : — 

 The dark markings, where composed of aragonite, are nearly 

 uniformly coloured throughout, except at the outer boundary where 

 contiguous to the granular calcite ; whereas in the other specimen 

 (fig. 2, p. 397), where they still retain much of the granular structure, 

 they are spotted all over with darker patches, as dark as the outer 

 boundary. 



The outer striated parts of the stone, which in one specimen are 

 of a different colour (see 7 & 9), 1 are exactly similar to the darker 

 striated portions contiguous to them, except in colour. 



The change of colour is in the compact aragonite, and not in 

 the granular calcite. 



The two colours, grey (inside) and yellowish-brown (outside), seem 

 to be very sharply defined, and to follow the striae when the stone is 

 viewed by the unaided eye : under the microscope this is seen not 

 to be the case, the change in colour is gradual, and the striae cut 

 indifferently through grey and brown. 



13. Thin veins of calc-spar in larger crystals are found, in what 

 are considered inferior kinds of Landscape Marble, and they may 

 cross the stone or follow the striae. These may often be seen quite 

 readily without the microscope. Edward Owen remarks (p. 180) 

 that these veins are broadest towards the bottom, and narrowest 

 towards the top. This is the case in my specimen ; indeed, the most 

 prominent crack does not extend far into the arborescent markings 

 (see SS, fig. 2). 



IV. Its Chemtcal Characters. 



14. The stone is violently attacked and rapidly dissolved by 

 dilute hydrochloric acid, with the exception of a small quantity of 

 brown argillaceous matter. 



The aragonite dissolves more rapidly than the granular calcite, 

 thus soon producing a very uneven surface. 



15. The stone consists chiefly of calcium carbonate, but the 

 solution in hydrochloric acid also contains iron, aluminium, man- 

 ganese, magnesium, and a little phosphoric acid. 



In addition to these there is the argillaceous matter and the 

 quartz, which are insoluble. By washing the former of these, and 

 examining it under the microscope, a few grains of the latter may 

 usually be found. 



It is also tolerably certain that there is some carbon associated 

 with the darker markings, to which in fact the colour is due, for the 

 stone is bleached on ignition. A white spot may be produced in a 



1 Numbers within parentheses, here and elsewhere in this paper, refer to the 

 numbered sections of the description. 



