10 PROCEEDINGS OF THE ISTATIONAL MUSEUM . vol.66 



COLEBAINITB 



A mineral having the optical properties of colerainite was seen in 

 a few thin sections as a miscroscopic mineral associated with mag- 

 netite along fractures. 



GARNET 



Garnet is an easily recognized microscopic constituent of the lime 

 silicate rocks where it forms minute sharply bounded euhedral 

 crystals. These vary from completely isotropic to rather notably 

 doubly refracting, with division into sectors. Some crystals have 

 an isotropic core with a birefracting border. The mineral also 

 forms irregular poikilitic areas in diopside rock which appear to the 

 unaided eye as black specks giving a "pepper and salt" appearance 

 in the hand specimen. 



Garnet forms large and^somewhat irregular areas associated with 

 magnetite along fissures. This garnet is nearly colorless, isotropic, 

 and devoid of crystal outlines. With it is associated another iso- 

 tropic substance of unknown character which has a golden brown 

 color and index of refraction below that of the garnet but still very 

 high. This brown mineral seems to replace the garnet and in places 

 tends to form globular masses, each of which contains what ap- 

 pears to be a minute colorless octahedron having the index of the 

 garnet. 



The most unusual garnet found in the quarries is obtained as a 

 coating on slickensides. Many of the small fissures along which 

 high temperature replacement of the limestone with diopside, mag- 

 netite, etc., has taken place are not healed but have been kept open 

 by sliglit movements which have produced slickensides. These 

 slickensides are coated, to an average depth of several millimeters, 

 with a green material which has all the appearance of serpentine, 

 which might be expected to occur in such manner. These were 

 thought to be serpentine in the field but the specimens of them col- 

 lected were found to be garnet when further examined. This gar- 

 net is so unusual in appearance and occurrence as to warrant a de- 

 tailed description. 



The coatings are associated with magnetite, diopside, etc., which 

 have developed in the adjacent rock and are usually well polished 

 by slickensiding. They have a pale serpentine green color, and are 

 dense with an opaline texture and waxy luster and subconchoidal 

 fracture. Like some amorphous minerals they tend to contract with 

 the formation of cracks which disintegrate them somewhat. The 

 material was found upon microscopic examination to be isotropic 

 with a refractive index above 1.82, the highest oil at hand. A pure 

 piece of the mineral was selected for analysis, ground and treated 



