MUSEUM OF COMPARATIVE ZOOLOGY. 9 
prehnite was discerned is No. 215 (Bell Rock, Malden) where it was 
found filling a minute fissure vein. The chalcodite which Professor 
Wadsworth has described was not noticed in any of the slides, and it 
seems certain that the important part which he assigned it, of completely 
taking the place of both feldspar and augite, is wrong. 
A typical and unusually fresh specimen of the diabase (No. 222, Pine 
Hill, Medford) has been subjected to a quantitative chemical analysis by 
R. C. Sweetser, B. S., Assistant in Chemistry at the Worcester Poly- 
technic Institute, to whom I would here express my great obligation. 
Though fully engaged with other duties, he kindly offered to do the work 
and obtained the results given below in column I. Column II. contains 
the results of an analysis of a diabase from the Lenneschiefer at Bichten- 
beck by Schenck, which shows considerable more decomposition, but is 
otherwise nearly identical.t Column III. is an analysis of diabase by 
Teall from Cauldron Snout, Durham, in the Whin Sill.” 
I, II. TET, 
SiO, 48.75 48.42 51.22 
Al,O5 17.97 17.59 14.06 
Fe,0, 0.41 1.05 4.32 
FeO 13.62 8.36 8.73 
CaO 8.82 7.73 8.33 
MgO 3.39 4.30 4.42 
MnO 0.91 — 0.16 
K,0 2.40 3.07 1.25 
Na,O 1.63 B15 2.55 
H,O 0.60 2.24 1.28 
TiO, 0.99 2.23 2.42 
P,0; 0.68 0.28 0.25 
Co, tr. 0.08 0.19 
FeS, tr. 0.15 0.49 
100.17 100.65 99.67 
Sp. Gr. 2.985 2.919 
Schenck considered orthoclase as probably present in II. The analysis 
as well as the extinction angle shows the feldspar to be more acid than 
that of I. Augite is changed to viridite, and ilmenite occurs and alters to 
leucoxene along the Gleitfldchen. The rocks I. and II. are thus shown to 
be very similar. The mineral composition as well as the chemical com- 
position of III. is also nearly identical with that of I. 
1 Adolf Schenck. Die Diabase des oberen Ruhrthals und ihre Contacterschein- 
ungen mit dem Lenneschiefer. Diss., Bonn, 1884, p. 20. 
2 Teall. Quart. Journ. Geol. Soc., XL. 640. 
