444 



Scientific Geology. 



scarcely be distinguished from hornstone, jasper, quartz, &c ; and 

 their fusion would be quite difficult. 



Now it seems to me that there is too close a resemblance between 

 this a priori reasoning and facts, to permit us to regard the former as 

 mere hypothesis. It obviously gives us a clew to the whole history 

 of compact feldspar, and prepares us to expect as great anomalies in 

 its characters as can present themselves. 



Compact feldspar in the vicinity of Boston forms deposits of con- 

 siderable extent: I mean that variety which is so deficient in crystals 

 of feldspar, that it would not generally be denominated porphyry. It 

 is true, however, that a careful examination of this rock, will almost 

 always detect these crystals in it; and sometimes polishing will bring 

 to light a porphyrite structure where it was not previously obvious. 

 Hence I have not hesitated to reckon compact feldspar as a variety of 

 porphyry. The variety most wanting in feldspar crystals usually lies 

 between genuine porphyry or sienite on the one side, and graywacke 

 on the other. A deposit of it thus situated, extends from Medford to 

 Maiden. Another strip of it may be seen in the south part of Dor- 

 chester and Roxbury, and in the north part of Milton and Ded- 

 ham. The same range probably, appears and forms hills in Need- 

 ham and Natick. It is most likely the other extremity of this 

 range that appears in Hingham. Often a portion of these deposits, 

 especially on that side where they unite with graywacke, exhibits 

 somewhat of a slaty structure ; and when describing graywacke, I 

 have mentioned a variety which is conglomerated compact feldspar. 

 This was noticed near Neponset river, not far from the line between 

 Milton and Dorchester. 



Although the compact feldspar under consideration assumes almost 

 every variety of color, yet there are certain predominant colors. One 

 of the most common is a grayish white : as in Medford, where some of 

 the rock has the aspect of granular quartz. This variety sometimes 

 assumes a yellowish tinge, as in Newbury; and this appears to be 

 one of the purest varieties of this rock, and the one, which, if any, will 

 answer for Turkey stones. (No. 1206.) Adarkgray color is another 

 that prevails ; and sometimes it is obvious that the rock embraces frag- 

 ments of indurated slate, not perfectly incorporated with the feldspar. A 

 more striking and very common variety is red, of various shades, 

 from brownish to blood red. The latter variety abounds in Hing- 

 ham, where ledges of it may be seen a little north of the^ village. 

 Specimens of this rock can hardly be distinguished from the jasper 



