WEATHEEIISra OF DIABASE 201 



indicating the presence of free carbonates, which are also observ- 

 able as secondary caleite when thin sections are examined under 

 the microscope. Some of this ealcite is evidently a deposit from 

 infiltrated waters, being derived from the surrounding decom- 

 posed material, while a portion results from the decomposition 

 of the silicate minerals in place. Aside from a slight taolini- 

 zation of the feldspars and development of chlorite from the 

 ferruginous silicates, there are no other observable signs of de- 

 composition, though the presence of a soda-bearing zeolite is indi- 

 cated by cubes of chloride of sodium, which separate out when 

 an uncovered slide is treated with a drop of hydrochloric acid. 



A glance at this table is sufficient to show that the disinte- 

 gration is accompanied by decomposition and a leaching action 

 which has resulted in the removal of a portion of the more 

 soluble constituents. The fact that the fresh rock yields the 

 larger percentages of its constituents to the solvent action of 

 acid and alkaline solutions is readily explained on this ground, 

 though it may be doubted if the full significance of the fact, so 

 far as it relates to siliceous crystallines, is as yet appreciated. 

 It will be observed that 36.23% of the fresh rock and 32.28% 

 of the decomposed is thus extracted. 



Of the material classed as silt in columns V, VI, and VII, or 

 as silt and clay, on p. 199, and which constitutes only some 

 3.17% of the entire residual debris, 77.87% is soluble in dilute 

 hydrochloric and sodium carbonate solutions. The insoluble 

 portion, constituting 22.13% of the silt, consists of unaltered 

 feldspar and iron, lime and magnesian silicates, which are easily 

 recognized under the microscope, in the form of minute, sharply 

 angular particles. Eecalculating, as before, the matter in col- 

 umns I and II on the basis of 100 and considering the alumina 

 as a constant factor, we obtain the results given in columns VIII 

 to XII inclusive, on p. 202, representing, so far as it can be ob- 

 tained by this method, the actual percentage loss of materials 

 attending the breaking down. 



From the figures in column X it appears that there has 

 been a loss of some 14.93% of all constituents. The increase 

 in water, as indicated by the ignition, is a natural consequence 

 of hydration and the presence of a small amount of organic 

 matter. This increase, it should be stated, is greater than may 

 be at first apparent, for the reason that the fresh rock contains 

 a considerable amount of secondary caleite, which is quite lack- 



