30 History of Hingham. 



History and otherwise, has expressed the opinion that much of 

 the pctrosilcx of the Boston Basin, and particularly the red rock 

 of Hingham, was derived from conglomerate. This view is not 

 held by others, whose opinions are entitled to respect ; but this has 

 not shaken confidence in his own. There is petrosilex, however, 

 in Hingham of quite a different character, but which he claims 

 has another origin. Mention of that will follow some further 

 remarks upon the red variety. 



Prof. Edward Hitchcock, in his great report upon the " Geology 

 of Massachusetts," mentions under the head of Porphyry the red 

 rock now under consideration, as occurring in Hingham in ridges 

 a little north of the village. Undoubtedly this accurate observer 

 found such ridges, though but one small exposure can now be 

 found above. the surface. This is near the junction of Crow-Point 

 Lane and Downer Avenue. Masses of this beautiful /ock may be 

 seen in the stone walls of Lincoln Street near Thaxter, and sug- 

 gest to the mind that in widening this street for the greater con- 

 venience of travel the ridges noticed were dcstroved. 



The rock is called above beautiful. Professor Crosby speaks of 

 it as the most beautiful of any in Massachusetts, and it undoubt- 

 edly is so. The color is a bright red, with interspersed spots of 

 lighter or darker hue. The variation was caused apparently in 

 some cases from the enclosure of pebbles, which, with the general 

 mass, became more or less fluent. The pebbly structure can be 

 better seen on weathered surfaces than on those caused by recent 

 fracture. 



The other variety of petrosilex referred to above, differs essen- 

 tially from the red, being of different color, rather more glassy in 

 lustre, entirely homogeneous, and presenting no appearance indi- 

 cating enclosed pebbles. Of the origin of this variety there can 

 be no question. It has the chemical constitution of granite, oc- 

 curs associated with it, and is undoubtedly the same with granite, 

 excepting that its mineral constituents are not crystallized, the 

 rock being too rapidly cooled to admit of crystallization. This 

 variety is always in Hingham associated with the granite. It may 

 be found with the granite that forms the cliffs of Peck's Pasture, 

 bordering the Home Meadows, and also on Lincoln Street, in the 

 rear of the first house next north of the Unitarian Church which 

 faces Fountain Square. Specimens from these and other localities 

 are in the collection of the Public Library, where may also be seen 

 those of the red variety. 



PORPHYRITE. 



The name Porphyrite has been given to basic rocks differing 

 but little in composition from Diorite and Diabase. Like them 

 they are composed of a triclinic feldspar with hornblende or 

 augite, but they are not, like them, crystalline granular. They 



