Johnson and Warren — Geology of Rhode Island. 27 



tions about 0"03 inm thick. This would indicate a double re- 

 fraction a little higher than the feldspar. 



The second substance is in part enclosed in the first in the 

 form of irregular patches. It replaces the first substance en- 

 tirely toward the margins and where the feldspar has entirely 

 disappeared it occupies the center of the area. With high 

 powers between crossed nicols it is seen to consist of an aggre- 

 gate of minute fibers and crystal plates often closely thatched. 

 Individual crystals have a distinct cleavage or lamination par- 

 allel to their length, to which the extinction is parallel and 

 which is also the direction of the vibrations of the faster ray 

 (A). The interference colors are a peculiar dull, dusty gray, 

 for - 03 mm thickness. With low powers the aggregates appear 

 almost isotropic. The characters of this mineral indicate 

 that it belongs to the chlorite family, probably near the 

 variety clinochlore. This identification has been repeatedly 

 confirmed by study of the same mineral, more coarsely crys- 

 talline, in the altered types described beyond, and is strongly 

 supported by chemical considerations (see later) as well as by 

 the occurrence of clinochlore in the veins which cut the rock 

 of the hill. The identity of the first mineral substance is not 

 so clear, but its easy passage into the chlorite as the alteration 

 progresses points to a very close relationship between the two. 

 The identification of chlorite, previously supposed to be serpen- 

 tine, adds not a little interest to the chemical aspects of the 

 alteration process. 



Enclosed in this pseudomorphic material, at or near the feld- 

 spar, are occasional irregular apparently isotropic bodies of a 

 brownish color and a high single-refraction. They have not 

 been positively identified. More or less actinolite in the form 

 of minute prisms and a few black particles are scattered 

 through the chlorite. 



About the spaces originally occupied by the feldspar, the 

 ore is more or less frayed out and finely granular, and clearly 

 shows the effects of alteration. Here too the olivine grains are 

 to a greater or less extent replaced by the chlorite. This 

 replacement begins at the borders, and while there is a well 

 marked tendency for the fibers to follow the direction of the 

 basal cleavage, their direction is often irregular. This chloritic 

 replacement is often either accompanied by, or immediately 

 followed by an aotinolitic one. The latter is strongest in the 

 neighborhood of the chloritic areas, but occurs to a variable 

 degree elsewhere. Olivine crystals surrounded by a border 

 of chlorite may be seen containing one or two well-developed 

 crystals of actinolite, or the crystal, within the border, may have 

 been wholly replaced by one, two, or a greater number of 

 actinolite crystals. The actinolite retains many of the original 



