Weatlicring of Diabase in Virginia, — Watson. 99 
laearing upon the relative durability of the various minerals 
constituting rocks is, however, quite conflicting and unsatis- 
factory." The results are probably, to a very large degree, 
controlled by local conditions. Cases are cited by professors 
Dana* and Merrillf among basic rocks, where the feldspar was 
the first mineral to yield to the atmospheric agencies. 
Amounts of Material Soluble in Different Strengths of Hydrochloric 
Acid. 
As the heading of table signifies, the results herein 
tabulated were obtained under as nearly the same conditions 
as was possible, varying only the strength of acid. In each 
case, approximateh' .5 of a gramme of the finely powdered 
rock was accurately weighed ofif and used. These weighed 
portions were digested in closel}' covered dishes with 100 C. C. 
each of the various strength acids, for three hours on a water- 
bath, brought to a strong ebulition and maintained through- 
out the digestion. They were then filtered and the filtrates 
subsequently evaporated to complete dryness in order that any 
soluble silica might be rendered insoluble. 
The acids used were carefully and accurately prepared from 
hydrochloric acid of 1.20 specific gravity. The strength 1:1 
by volume, meaning 50 C. C. of 1.20 specific gravity acid to 
50 C. C. of distilled water — at ordinary temperature. The 
normal, half normal, and quarter normal strengths, meaning 
the whole, half and quarter molecular weight respectively of 
1.20 specific gravity acid dissolved in one litre each of dis- 
tilled water. 
While the results, probably, have not been carried far 
enough to warrant any very definite statement, thev are, I 
believe, highly suggestive and of some interest. No limit was 
reached where the extracts remained approximately the same 
for any two acids of nearly equal but varying strengths, but 
the difference in each case is appreciable and quite noticeable. 
It would be expected that where all other elements of con- 
dition remained fixed and constant, that a point would be 
reached where a maximum extract would be found, corres- 
ponding to a certain strength acid. 
*Rep't. Wilkes Exploring Expedition, Geology, 378. 
tBulletin U. S. G. S., No. no, 1894. 
