Johnson and Warren — Geology of Rliode Island. 7 



due to glacial action and many fine examples of glacial mark- 

 ing may be noted over most of the surface. 



The hill is traversed by a nearly vertical system of main joints 

 running nearly south and north. Roughly perpendicular to 

 these is a system of minor joints which cut the hill up into 

 small, rudely prismatic blocks. On the eastern side of the hill 

 the minor jointing is rather thinly parallel in places. Many 

 blocks have been torn out and carried away by the ice, leaving 

 troughs and irregular breaks in the surface. 



At several points along the exposed ledges on the western 

 side, distinct inclusions of a feldspathic rock may be seen. 

 These vary in size, so far as observed, from two to six 

 inches in diameter. They are always much rounded in out- 

 line, evidently from alteration, and are surrounded by a 

 variable amount of green chloritic material. Macroscopically, 

 they resemble the gabbro noted beyond as being the nearest 

 rock to the cumberlandite on the west, and microscopically, 

 they were found to be practically identical with it. On a 

 recently uncovered, glaciated surface at the northeast end of 

 the hill, two subangular inclusions composed of a compact, 

 greenish grey mass of chloritic material mixed with some 

 actinolite and magnetite, were observed. Under the micro- 

 scope indications of some previously existing, rather coarse 

 textured rock were observed, and it is thought that these 

 inclusions may also be gabbro, now strongly altered like the 

 inclosing rock. 



These inclusions, now noted for the first time as belonging 

 to the gabbro, are the only evidence that the cumberlandite is 

 ■younger than a portion at least of the adjacent igneous rocks. 



The rock has suffered to some extent from shearing move- 

 ments which have produced a schistose structure in the actin- 

 olite and chlorite commonly found along the joint planes. 

 Many of the joints contain crystals of these minerals of con- 

 siderable size, and weathered faces are sometimes thickly 

 studded with well-developed crystals of clinochlore. Less com- 

 monly crystals of a dark glassy, amber colored olivine, the rare 

 species hoi'tonolite, are associated with the other two minerals. 

 Veins, sometimes as much as an inch in thickness, containing 

 varying proportions of these three minerals, cut irregularly 

 through the rock. 



As a whole the hill has resisted the action of weathering to 

 a remarkable degree considering its mineral character. Its 

 surface is not decomposed in general to a depth of over one- 

 half an inch, and Professor Shaler's observation that the sur- 

 face of the hill has probably not lost more than six inches by 

 weathering alone since glacial times, seems entirely probable. 



