MUSEUM OF COMPARATIVE ZOOLOGY. 9 



prehnite was discerned is No. 215 (Bell Eock, Maiden) where it was 

 found filling a minute fissure vein. The chalcodite which Professor 

 Wadsworth has described was not nut.'ced 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 fuUy 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 Bochteu- 

 beck by Schenck, which shows considerable more decomposition, but is 

 otherwise nearly identical.^ Column III. is an analysis of diabase by 

 Teall from Cauldron Snout, Durham, in the Whin Sill.'' 



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 Gleitjldchen. 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- 

 angen mit dem Lenneschiefer. Diss., Bonn, 1884, p. 20. 



2 Teall. Quart. Journ. Geol. Soc, XL. 640. 



