Griswold.] 420 [Dec. 19, 
marked result. As both of these tests fail to denote that novacu- 
lite is chalcedony the writer feels that the possibility presented 
by the comparison of specihc gravities must be rejected. 
From the occurrence of the rhombohctb-al cavities in the rocks 
from Arkansas Mr. Rutley finds much support for his argument. 
He considers them in the early condition when tilled by carbonate 
as well as in their present condition. The early carbonate he 
believes to have been a dolomite, because dolomite more com- 
monly takes the rhombohedral form than does calcite. To this 
idea is opposed the fact that rocks of the series associated with 
those containing cavities contain crystals which gave a lively 
effervescence with cold dilute hydrochloric acid and were 
regarded as calcite.^ For this reason the calcite idea was the one 
favored in the " Report," and the word dolomite was introduced 
merely as precautionary. 
The cavities are thought by Mr. Kutley to give further evidence 
of a dolomitic origin by their departure in minor irregularities 
from perfect rhombohedral form, the conception being that a 
dolomitic rock in its last stages of replacement by silica would 
present scattered patches of dolomite having such an irregular 
outline. It would seem to the writer that the cavities were too 
constant in size and form to admit of such an explanation.'^ The 
point, however, is an important one and cannot be dismissed by 
a mere statement of opinion, because it is an evidence by which 
Mr. Rutley infers a former greater quantity of the carbonate. 
That Mr. Rutley's explanation of the slight irregularity in form 
of the cavities is the necessary one is not demonstrated ; the 
writer would still claim consideration of his early idea that '•'• some 
granular sihca like that of the groundmass of the rocks has been 
either forced into some cavities or dej^osited there from solution 
so that the rhombic outUne is often obscured.^ " 
Again, the somewhat orderly arrangement of silica grains about 
the cavities, the longer axes of the grains tending to take a 
position perpendicular to the side of the cavity, is regarded by 
Mr. Rutley as evidence that the carbonate existed before the 
deposition of the silica. If the silica is deposited around the 
1 Opus C'it., p. V.il. 
^Ojms(nt.,\>.Hyi. 
^ Opus eit., p. 187. 
