142 M. Godon’s mineralogical observations. 
could supply the whole United States with this kind of mineral, which 
is sold at a pretty high price in commerces To this species I attach 
the name novacular. 
24. ‘The naked eye, and still better the magnifying glass, discoy- 
ers, in almost all the above described species of petrosilex, small par- 
ticles of simple minerals; such as quartz, epidote, amphibole, small 
crystals of felspar, arsenical and simple sulphurated iron, &c. The 
grains of quartz appear particularly abundant, when it adjoins to the 
wacke. A variety is observed [Brushhill turnpike, ] which falls into 
pieces, the faces of which are coated with dendrites of black manga- 
nese, which is rendered very apparent by the almost white field of 
the stone. 
Simple petrosilex abounds principally on the north and south of 
Boston, where it sometimes forms hill It is observed contiguous to 
wacke and felsparoid, where it appears confounding itself with these 
two rocks. [Quincy O, Malden a, Dorchester a, Dedham mr]. 
2. Porphyritic petrosilex. 
25. Most of the petrosilex above mentioned [Quincy n, Dorches- 
ter 0], presenting some chrystals of felspar scattered in a whitish or 
brownish cement, might be considered as porphyries ; but, agreeably to 
the common meaning we shall restrict this term to tle kind, which pre- 
sents crystals of felspar in a notable proportion, cemented by a petro- 
siliceous substance, commonly of a dark colour, as reddish, brown-red, 
ne black, &c. 
- The reddish brown variety, with white felspar, is the most 
ares It forms vast masses [Malden y, Lynn. rj, contiguous 
to felsparoid and jasper petrosilex, which evidently constitute its ce- — 
ment, Some fragments of considerable bulk are found, of a deep red 
