MM. Godon’s mineralogical observations. 143 
color, and equal in beauty to the best antique prophyry [Chelsea O, 
Lynn O]. This variety sometimes contains crystals of amphibole, 
and some particles of a deep green colour, which seem to belong to 
epidote. 
27. Some loose fragments. of porphyritic petrosilex are found. of 
a greenish and black colour [Chelsea O, Islands of the harbour 0}, 
This last, which presents white shining crystals of felspar, appears 
analogous to the black porphyry of the ancients, porfido nero of the 
Ttalians. Perhaps.it is but a variety of porphyritic amphiboloid. A 
variety is found in the northern part of the Blue hills, presenting small 
crystals of a light red colour, cemented by a dark brown petrosilex, 
fusible into a black enamel. It is observed running across the main 
rock of this part of the country.* 
28. The surface of petrosilex, simple and porphyritic, eigibaed to 
the contact of the air, moulders, and presents almost always an earthy 
crust, sometimes adhering to the tongue. _In its-primitive state, how-: 
ever, this rock is commonly so hard as to sparkle with steel, and then 
it is susceptible of a high polish. This polish protects it against de- 
* The Blue hills are in Milton, about ten miles south from Boston, and are the 
highest ground in this part of Massachusetts. From the most elevated, as from am 
observatory, an extensive portion of this picturesque country is presented. to the 
view. They appear to consist of felsparoid, verging to petrosilex, simple-and 
porphyritic. This rock is often interrupted by. powerful veins of quartz, full of 
cavities, coated with small crystals of the same substance, some particles of oligist 
iron, and signs of carbonated iron (brown spar), 
The observations, made in this part, are common to the Hon. Judge Davis, 
Messrs. Joseph Tilden, Richard Webster, T. Capt, and Charles Davis. I hope 
these gentlemen may give more extent to the yet imperfect des¢ription of those’ 
interesting heights. 
