IGNEOUS KOCKS 



471 



Diamond Hill quartz deposits. — This hill is undoubtedly the most 

 striking geologic feature of the whole area. On the west the hill rises 

 abruptly from a fiat valley to a height of 280 feet. Its western top forms 

 a jagged, picturesque ridge, barely covered by a scanty growth of small 

 brush. The whole hill covers about one-third of a square mile, and of 

 this at least one-third is either quartz or highly silicified felsite. To the 

 east of the ridge, which is quartz, the hill slopes off rather gradually and 

 is made up of felsite more or less cut by quartz veins for some distance 

 away from the main center of silicification. The hill was briefly referred 

 to by Woodworth in his report on the I^arragansett Basin and was made 

 the subject of a careful study by Messrs. K. A. Barber and H. S. Mears.*- 



The western third of the hill is practically a replacement of felsite by 

 quartz largely in the form of veins. These vary in width from those of 

 microscopic dimensions to those 5 inches in vndth, and many of the 



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Narragansett series 



Labradorite porphyry dike 



KXJ( 



^7V- 



Riebeckite granite with porphy 

 ritic marginal facies 



Grants Mills granite 



^ Ashton schist 



Cumberland quartzite 



Figure 3. — Geological Section through Cumherland Hill 

 See BB on map, page 437 



larger ones are of great length. While these have a very distinct trend 

 parallel to the longer axis of the hill, they may be found running in all 

 directions and, with innumerable ones of smaller size, form a most intri- 

 cate and amazing network of veins. Their directions seem to have been 

 determined by the jointing and fracturing of the rock. 



The quartz is chiefly of the milk-white variety. In the veins, the crys- 

 tals grow perpendicularly to the walls, giving rise to comb structures and 

 very often with central vugs. Id these the crystals are beautifully termi- 

 nated and, as tlie rock commonly breaks along the line of these vugs, 

 natural surfaces are covered with a mass of small quartz crystals which 

 glisten like diamonds in the sunlight — a phenomenon that has given the 

 hill its appropriate, though to the non-mineralogical mind a misleading, 

 name. There are often successive layers of crystals separated at times by 



Unpublished thesis. Mass. Inst. Technology, Boston, Mass., May, 1906. 



