Johnson and Warren — Geology of Rhode Island. 35 



They are of a dark green to almost black color. Many of them 

 are twinned after the characteristic manner of the chlorite group. 

 Under the microscope the crystals show a strong pleochroism, 

 brownish red parallel to c' (C), dark emerald-green perpendicular 

 to c' . The crystals show an optically positive interference figure 

 with an axial (2E) angle varying considerably but running as high 

 as 23 degrees according to an approximate measurement made 

 with the aid of a Schwarzmannische Axenwinkel scale. Several 

 exposed and strongly rusted joint surfaces are thickly studied 

 with these clinochlore crystals. In such cases the other minerals 

 have either entirely weathered away or were originally absent. 



The Hortonolite. — In examining one of the best exposed of 

 the veins, situated on the western side of the hill somewhat 

 south of the center, Professor Charles Palache, of Cambridge, 

 discovered a dark resinous-looking mineral associated with the 

 actinolite and clinochlore which he correctly referred to the 

 olivine group and which subsequently proved on analysis 

 to be hortonolite. The material collected by Professor Palache 

 was very generously placed by him at the writer's disposal for 

 investigation and has proved the best so far obtained. No 

 crystallographic outlines have been noted on the hortonolite 

 and its relations to the other minerals are distinctly xeno- 

 morphic. It possesses two well-marked cleavages at right 

 angles to each other, and, when fresh, small fragments have a 

 light yellow color by transmitted light. The mineral tends to 

 decompose readily to a reddish brown, ferruginous earth with 

 an accompanying discoloration of the actinolite and eventual 

 disintegration of the vein. An examination of a large number 

 of specimens, collected from different veins, shows that the 

 hortonolite is very generally present, and when not actually 

 seen the characteristic ferruginous alteration material indicates 

 that it was originally present. It is, therefore, to be looked 

 upon as a normal constituent of the veins of the hill. With 

 the aid of a miscroscope a sufficient number of clear yellow 

 particles were picked out with considerable labor from some 

 of the larger fragments, and analyzed. The results are given 

 in column II. The specific gravity was found with the pycnom- 

 eter to be 4*054 at 20 degrees C. 



The ratio Si0 2 : RO is very nearly 1:2, the olivine ratio, 

 and the composition is close to that of the hortonolite from 

 Munroe, Orange Co., ~N. J., analyzed by Peniield and Forbes* 

 and given in column III. The occurrence of this rare mineral 

 species at a new locality as a distinctly secondary mineral in 

 veins traversing an ultrabasic olivine rock is of special interest 

 to mineralogists. The occurrence of a member of the olivine 

 group as a vein mineral has been noted and described by E. 



*This Journal, No. 151, p. 132, 1896. 



