J. D. Dana — Chloritic formation west of New Haven, Conn. 121 

 beds, i 



studied for \ 



The resemblances of the massive rock to trap was long since 

 noticed by Professor Silliman, wbo, in a paper on the geology 

 of New Haven and its vicinity, published in 1811 in Brace's 

 Mineralogical Journal, called it " primitive greenstone." In fact, 

 the similarity in external aspect is so close that hand speci- 

 mens from some portions of it would without question be 

 pronounced trap — that is, doleryte, diabase, or melaphyre— by 

 the most experienced lithologists. 



In 1872, an incomplete analysis of the feldspar in the pale 

 grayish green porphvritic rock, outcropping just south of 

 Maltby Park, was made, by Mr. Edward S. Dana. The amount 

 of silica afforded by the feldspar having been found to be 

 but 45 per cent, the conclusion suggested was that the rock 

 consisted largely of labradorite, and that it was probably essen- 

 tially identical with part of the trap of the Connecticut valley 

 dikes. In view of the presence of chlorite, I hence regarded 

 the compact rock of the region as a metamorphic diabase ; and 

 it is the rock specially referred to under that name in the last 

 edition of my Manual of Geology, 



Still, the analysis, besides being incomplete, was not satis- 

 factory because the feldspar (-rystals, although of the normal 

 hardness, were granular in texture, without good cleavage, sug- 

 gesting that they might possibly have undergone a partial alter- 

 ation. On account of Mr. Dana's departure for Europe, he was 

 compelled to leave the investigation he had begun unfinished ; 

 and so it has remained until this summer, when it was taken 

 up, at my request, l)y the skillful analvst connected with the 

 mineralogical department of the Sheffield Scientific School of 

 Yale College, Mr. George W. Hawes. His results prove that 

 the rocks are in fact metamorphic doleryte, meiamorphic diohase, 

 and metamorphic melaphyre; the first two, labradoritc locks, 

 and the last an oligoclase variety. To distinguish tliese meta- 

 morphic rocks from the igneous of the same composition, they 

 are named, on my suggestion, metndoleryte, metadiahase, and 

 metamelaphyre. The examples are part of a long series of rock 

 species which have representatives both among igneous (or in- 

 trusive) and metamorphic rock.s. Other kinds are dioryte and 

 metad'oryte syenyte and metasyenyte, fehyte and metafehyie, etc. 



We have here the important geological fact that lahradorite 



is a prominent constituent of certain metamorphic rocks which 



have the aspect of much dioryte, and which are probably of 



Lower Silurian origin.* The labradorite — a lime-and-soda feld- 



• On the question of their age I have collected many facta and propose before 



