ox 



dS MERKILL DISINTEGRATION AND DECOMPOSITION OF DIABASIO. 



less, tlie total loss of materials amounts to but 13.46 per cent, witli gains 

 in volatile constituents as before. Of the individual constituents, 28.62 

 per cent of the original soda ; 31.98 per cent of the potash ; 25.21 per cent 

 of the lime; 40 per cent of the phosphoric acid; 14.89 per cent of the 

 silica, and 3.23 per cent of the alumina have disappeared. 



It need scarcel}'' be observed that these results cannot be considered ab- 

 solutely satisfactory. The fact that it is found necessary to change our 

 basis for calculation as occasion demands, from the alumina to the iron 

 oxide, in order to avoid an actual and presumably impossible gain in 

 certain constituents,* shows at once that the method is liable to serious 

 error. Moreover, as in all such cases, any slight change in the figures of 

 the analysis is exaggerated in those of the resulting calculations. I be- 

 lieve such errors to be, however, always on the side of too low estimates 

 of the material lost, and in spite of the acknowledged inaccuracy, it seems 

 to me to be one yielding very interesting and, when proi)er allowances 

 are made, instructive results as well. 



Time Limit and Extent of Disintegration. 

 As was the case with the disintegrated rocks of the District of Columbia, 



we are enabled here also to fix a geological limit to the beginnings of dis- 

 integration. As above noted, the dike occurs in a region of extensive 

 glaciation. This is so evident and well known as to need no confirmatory 

 statement. That the disintegration and decay into which the rock has 

 fallen is subse(![uent to the glaciation, and is not an isolated case of pro- 

 tection from erosion, as might at first be thought, is shown by the presence 

 of glacial stride still traceable over the surface of portions of the decom- 

 posed dike and the deposit of till overlying it, as shown in the plate. It 

 is, of course, possil)le that decomposition had set in prior to the i)eriod 

 of glaciation. That the process had not gone on extensively, however, is 

 evident from the fact that the material was still sufficiently firm to receive 

 the glacial markings. We are apparently safe in assuming that this dis- 

 integration and decay, or degeneration, as I have called the combined 

 processes, and which extends to a depth of 30 feet, or perhaps 50 feet or 

 more along joint plains, is mainly postglacial.f That the degeneration 

 has here gone on more extensively than in other dikes of the vicinity is 

 due, as the writer believes, to its coarse and somewhat granular structure, 



'•'The tables for the Venezuela diabase, when recalculated on an alumina constant basis, showed 

 the total loss of all constituents to be 33.08 per cent, with an apparent gain of 3.3.5 per cent in iron 

 oxides. This gain is doubtless due in part to a change in condition from FeO to Fe203. The 

 granite analyses, similarly calculated, showed a total loss of 9.74 per cent, with an apparent gain 

 of 1.15 Fe203 and 0.05 per cent MgO. 



t This is the conclusion also reached by Crosby. Technological Quarterly, vol. iii, no. 3, August, 

 1890, p. 237. 



