Weathering of Diabase ifi Virginia. — Watson. 
87 
shown in this locahty. Between the two extremes are noted 
many stages of decay. Boulders of variable sizes and in differ- 
ent stages of decay are imbedded and distributed throughout 
the mass of incoherent red clay. From the beginning in 
weathering to the stage where the minerals are entirely de- 
composed, but still held intact, the layer of shelly structure is 
well shown in the boulders. 
While the color of the decay is prevailingly red, frequently 
a mottled gray color is seen, and is especially noticeable in 
the boulders, which have not reached the limit in decomposi- 
tion, but those in which the shelly structure is still shown. 
PetrograpJiic Features of the Roek. 
TABLE NO. I. 
Bulk analysis of Fresh and Disintegrated Diabase from 
Chatham, Virginia. 
CONSTITUENTS. 
Silica SiOa 
Alumina AUOg 
*Iron FeoOg 
Lime CaO 
Magnesia MgO. 
Soda NagO 
Potash KgO.... 
Water HgO.... 
I 
II 
III 
4S 
^^ 
n 
4« 
II 
60 
9 
92 
15 
40 
3 
24 

47 

94 
47.87 
14-43 
11.55 
10.45 
10.58 
3-47 
0.61 
1.82 
IV 
37 
09 
13 
19 
35 
69 

41 

57 
I 
75 

33 
II 
83 
52.06 
13.67 
15-97 
8.15 
5.01 
3-36 
0.86 
1.05 
100. 13 
•2.953 
Total 100.78 100.78 100.86 
Specific gravity 3 . 026 
I. Fresh olivine diabase rock. 
II. Partially weathered olivine diabase rock. 
III. Decomposed olivine diabase rock. 
IV. Fresh quartz diabase rock. 
The fresh rock (see i in the above table) is a coarse 
granular admixture of plagioclase feldspar, augite and olivine, 
with accessory magnetite. Biotite, chlorite and serpentine 
occur as secondary minerals resulting from alteration. The 
structure is characteristically ophitic, showing in a marked 
degree the lath-shaped feldspars with the interstices filled 
*A11 the iron is calculated as FegO.,, which necessarily increases the 
summation of the results, inasmuch as some ferrous iron is present in 
the rocks. 
