Weathering of Diabase in Virginia. — Watson. 91 
which is a result from the breaking down of the iron in the 
oHvine. 
No. 3 (See bulk analysis, page 87) is the completely 
weathered product of No. i, which under the microscope is 
seen to be a highly ferruginous mass in which some silicate 
minerals can be noticed but not identified. However, after 
removing the iron stains by dilute acid, undecomposed grains 
of feldspar, augite and magnetite are clearly recognizable. 
No. 4 (See bulk analysis,page 87)is from an adjacent dike, 
and differs somewhat from No. i in mineral composition. It 
is a fine-grained rock with a somewhat less characteristic 
ophitic structure. The essential minerals are augite and 
plagioclase feldspar with considerable quartz, which is prob- 
ably in quantity sufficiently large to warrant the name "quartz 
diabase." The accessory minerals are hornblende, magnetite, 
biotite, pyrite, chlorite, serpentine and calcite. 
Probably the most interesting feature of this rock petro- 
graphically, is the presence of an intergrowth of quartz and 
feldspar, forming a micrographic structure. The same 
structure has been noticed by Dr. Adams* in some of the 
Canadian diabases found to the north of the Island of 
Montreal. 
Mechanical A?ialysis of Decayed Rock. 
In orderv to more accurately indicate the physical con- 
ditions of the decayed product, and at the same time, point 
out to some degree, the physical changes in the degeneration 
of this rock, a complete mechanicalf analysis has been made 
yielding the results given below. 
The material analyzed does not represent the final decayed 
product, or soil, but is the outer decayed portion of one of 
the small boulders found scattered through the mass of in- 
coherent clay. The decayed portion was still coherent, fairly 
firm and intact, but crumbled with considerable ease under 
pressure in the hand. In color, it was a bright orange yellow 
with a striking resemblance to one of the bright yellow colored 
*See, Report on the Geology of a portion of the Laurentian area ly- 
ing to the north of the Island of Montreal, Geol. Sur. Canada, Annl. 
Rept. 1896, part J, Vol. VIII, pp. 136-137. 
tl am indebted to Mr. R. S. Hosmer of the Division of Soils, U. S. 
Department of Agriculture, for the analysis. 
