30 Johnson and Warren — Geology of Rhode Island. 



type, and the chloritic border between the ore and the olivine 

 is more pronounced than before. Minute veinlets, similar in 

 character to the larger veins of the hill, are occasionally seen 

 macroscopically and in the slides. The accompanying figure 

 (3A) will illustrate the relations of the various minerals as 

 seen in thin section. 



Actinolitic Type. — By a considerable increase in the amount 

 of actinolite, we pass into what may be called the "actinolitic'''' 

 type. So far as quantity goes, it is ranch less important than 

 any of the other types. It occurs locally in irregular patches 

 or streaks through the hill. Compared with other types, speci- 

 mens of it are lighter and more yellowish in color and possess 

 also more glistening surfaces, by reason of the presence of 

 abundant actinolite crystals of a pale yellowish green color and 

 with highly developed cleavages. The ore is much less con- 

 spicuous as a rule. In fact, in extreme cases it is hardly 

 noticeable, and the rock then consists practically of actinolite 

 and the compact chloritic material. The chloritic areas 

 seem to be rather more abundant, somewhat less distinct in 

 outline, and show a tendency to merge into one another. The 

 actinolite crystals rarely exceed 2 or 3 mra in length. 



In thin section the green areas appear much the same as in 

 the previous type. In their neighborhood the olivine is 

 strongly replaced by chlorite as before. The actinolitic 

 replacement, however, reaches a maximum development 

 and frequently displaces all of the olivine. The ore-matrix, 

 although considerably broken, preserves very well its original 

 outlines. The chloritic borders about the olivine grains 

 persist even where the olivine has been entirely replaced by 

 the actinolite. Whether or not the latter mineral is affected 

 by the chloritic alteration has not been definitely made out, 

 but if such is the case, the process has gone on only to a very 

 slight degree. Except in extreme cases the texture of the 

 rock in section differs but little from that of the preceding 

 type, and in general appearance they are also similar — 

 actinolite lying in the place of the olivine. 



The actinolite possesses a very pale greenish pleochroism and 

 a rather high extinction angle, 20 degrees measured on the pris- 

 matic cleavage cracks. It was found by comparison (immersion 

 method) with the vein actinolite described beyond that the two 

 possess practically the same index of refraction. Fragments 

 of the actinolite from the rock sank in the Thoulet solution a 

 little more readily than that from the veins, although this may 

 be accounted for by the presence of included ore particles 

 without which the mineral from the rock could not be obtained. 

 For this reason no attempt was made to analyze the mineral. 

 The rock and vein minerals appear, however, to be essentially 



