The Geology of Hingham. 29 



mentioned, except a limited one on Central Street, between four 

 and five hundred feet from Elm Street, in a field adjoining the 

 west side of the road, and two others of small area on a field at 

 the corner of Central and Elm streets. Away from usual travel 

 between Fort Hill Street and Weymouth River the rock appears 

 in numerous exposures. Reference to the map will give their 

 location. 



MIXED ROCKS. 



The rocks so-called by Professor Crosby, though simply com- 

 posed of a mixture of the two kinds already described, are of such 

 peculiar combination as to seem worthy of notice under a separate 

 heading. There is no appearance among them, as far as observed, 

 of anything like a dike of either penetrating the other. There is 

 found simply a mixture of masses of every size and shape, each 

 single mass being clearly distinctive as granite or diorite, the ele- 

 ments of one in no case coalescing generally with the other. The 

 locations of these rocks have been mentioned in the remarks upon 

 the diorite. 



There seems no way of accounting for such mixture except by 

 supposing that at the time of their eruption the rocks existed sep- 

 arately beneath the surface in two contiguous zones, both being in 

 a plastic condition, and that when forced to the surface they were 

 made to intermix so as to present themselves as now found. 



PETROSILEX. 



The rocks of Hingham hitherto known as porphyry, compact 

 feldspar, and felsite, the writer classes under the name of petrosi- 

 lex, as with but one or two exceptions to be mentioned, all belong 

 to that division of such rocks as contain over 63 or 64 per cent of 

 silica, and which Phillips and others have designated as petrosilex, 

 retaining the name felsite for those of a more basic character, and 

 having a plagioclase feldspar instead of orthoclase as a constituent. 



The name " porphyry " is no longer in use as a substantive by 

 geologists. It was applied by the ancients to rocks generally ho- 

 mogeneous, but which contained crystals, commonly feldspar ; and 

 this use continued to modern times. As, however, the rocks so- 

 called differed widely in composition, and it became necessary in 

 the progress of science to define their character more particularly, 

 the name became obsolete. The word " porphyritic," however, 

 remains in common use as an adjective expressing the texture of 

 rocks of a homogeneous base, having crystals disseminated through- 

 out their mass. Thus petrosilex with enclosed crystals is called 

 porphyritic petrosilex, and diabase, the rock of trap dikes with 

 enclosed crystals, is called porphyritic djabase or porphyritic trap. 



The writer, in communications to the Boston Society of Natural 



