26 History of Hingham. 



tions, no exposure of it having been observed in all the region 

 south of Hingham Centre. This rock contains necessarily but 

 one constituent, a triclinic feldspar, usually oligoclase, but it has 

 generally associated with it hornblende. Sometimes mica is also 

 found in it and not infrequently particles of quartz. 



In Hingham, when composed of feldspar and hornblende in 

 nearly equal proportions and when the grains of each are clearly 

 perceptible, it appears not unlike granite, but having no quartz as 

 a general constituent it may be readily distinguished. In limited 

 areas it is found almost entirely of feldspar, when it presents 

 itself simply as an impure white rock, its character being conse- 

 quently more obscure. 



One of the best exposures on a highway of the town of typical 

 diorite occurs in Summer Street, on the right side going south 

 from the railroad crossing, and within 100 feet of Kilby Street. 

 It juts into the road from the adjoining field, and presents itself 

 with a smooth, rounded face about twelve feet across, on which the 

 two minerals, feldspar and hornblende, are well defined and plainly 

 visible. Two other exposures may be seen between the one men- 

 tioned and Kilby Street, and in the field back from the road are 

 several ridges of it. 



Another interesting exposure of diorite on a highway is to be 

 found on the surface at the top of Fort Hill, just front of the 

 cemetery. Here it is cut through by numerous narrow veins of 

 a whitish granite, which by distortion and separation of parts 

 afford an interesting studv for the observer. The rock of this 

 locality should not be disturbed, as there is no other known 

 instance in town where granite can be seen so clearly to have 

 been intruded in veins into the diorite. The exposure here, too, 

 is interesting from the glacial striae which may be plainly seen 

 upon its surface. 



Diorite occurs abundantly on East, Kilby, Weir, and Hull 

 streets. 



Going northeast from Horticultural Hall on East Street, some 

 rocky elevations appear on the left side of the road which are 

 known as Andrew Heights. The rocks of the slope facing the 

 street are diorite, with the exception of an intervening portion 

 of granite. This last rock also appears on the land opposite the 

 diorite back from the road. Beyond the heights mentioned, all, 

 or nearly all, the rocks of the street and of land contiguous are 

 diorite until Kilby Street is passed. 



Intermediate between Andrew Heights and Kilby Street, by a 

 reduction of the level of the road over an elevation, and the neces- 

 sary excavation of rock, there is left exposed on the left side a 

 cliff of considerable interest. The main body is a dark diorite, 

 but there may be seen by close examination a distorted dike of 

 felsite eight or ten inches in width, and a mass of diabase trap, 

 both of which have been intruded into it. The trap contains an 



