LITHOLOGY. 1 57 



said of those specimens which have been determined, that the anorthite 

 varieties have crystals that are short, thick, and well defined in outline, 

 while the crystals of labradorite are long and irregularly terminated. 

 If this should be regarded as characteristic, both varieties are present 

 among these more decomposed rocks. 



Olivine Diabase. Ever since Prof, O. P. Hubbard found the remarka- 

 ble boulders of this rock at Thetford hill, those interested in them have 

 been hoping to find the rock in place. The employment of the micro- 

 scope brings these interesting rocks to light, and the specimens, though 

 not so remarkable when examined with the unaided eye, are, when cut 

 into sections and magnified, found to be very interesting and beautiful. 



The boulders found at Thetford hill are composed of large round 

 masses of olivine sometimes two inches in diameter, large rough greenish 

 plagioclase crystals, and large black augite crystals, all embedded in a 

 small amount of a ground mass. Dr. Hunt has described a rock exactly 

 like this, which is in place at Montarville, in the neighborhood of Mon- 

 treal. From whence these boulders came is not known. 



Olivine bearing diabase is not a common rock, though it seems quite 

 plain, as Mr. Rosenbusch remarks, that microscopic studies will much 

 increase the number of its representatives. As found in dykes at Camp- 

 ton falls, it is a black porphyritic rock. The macroscopic crystals are jet 

 black, and with the unaided eye it would be hard to say whether they 

 were of augite or hornblende, as they are not large, and show no distinct 

 cleavage. They have, however, very well defined outlines, indicating a 

 good crystallization. The olivine is not distinguishable as such, and the 

 reason becomes very plain when the sections are studied : it is because 

 they are no longer olivine. In the thin sections, we see that in a compact 

 and very fine mixture of crystals of a triclinic feldspar, augite, biotite, horn- 

 blende, chlorite, and magnetite, are larger and well formed crystals of a 

 triclinic feldspar, augite, and olivine. The olivine is very well crystal- 

 lized, and a section through two of its crystals is represented in Fig. 4 on 

 PI. 7. This olivine is all much altered. The centres of the crystals are 

 in some places intact, but most of them are entirely changed into a 

 greenish-fibrous serpentine-like mineral — a kind of alteration to which 

 olivine is peculiarly liable. The augite is quite abundant, and its large 

 crystals are perfect in outline, but the fine augite scattered through the 



