On the Geology of the Key of Sombrero, W. I. 269 



Magnesia 2.46 1.92 



Alumina 1.89 3.75 



Protoxide of Iron 3.08 2.66 



Alkaline chlorides 2.36 1.81 



98.99 99.13 



The greenish color is due to the Protoxide of Iron, as the 

 insoluble silicate is a light brown impalpable powder. It yields 

 up its bases to boiling sulphuric acid, and is also partly decom- 

 posed by mere ignition. 



The long and finer veins are generally occupied by the first 

 kind of cement; the shorter, either by the first or second. 

 The latter often do not have vertical straight sides, but have 

 evidently been widened before the reception of the marl, which 

 fills many rounded expansions in their sides. Many of the 

 longer veins, which are sufficiently wide at their origin and 

 become narrower as they descend, are occupied by conglo- 

 merate above and cement below. Sometimes where a vein 

 intersects the bedding-line of B or A, the conglomerate has been 

 interlaminated for some distance around between the beds. 

 The broader veins are occupied by nodules from the size of 

 gravel up to six or nine inches in diameter, with their longer 

 dimensions arranged vertically, as they have fallen or worked 

 their way down the fissures. In the basins the cement predo- 

 minates, consisting usually of the third kind. In many basins 

 the sides of the slope and the bottom are covered with the 

 largest blocks ; above these the pebbles of the conglomerate 

 decrease in size upwards ; and the highest portion of the cavity 

 is filled merely with the cement, arranged in horizontal laminae. 

 Sometimes the whole basin is not completely filled up. Traces 

 of the green marl may sometimes be seen among and beneath 

 the pebbles on the surface of the bed, or intermingled with the 

 material of the subjacent bedding-line. The fossils of the peb- 

 bles are identical with those of beds C and D. In the cement, 

 however, a very few individuals of Trochus and Cerit Mum (but 

 no corals) have been observed. 



