FRAGMENTAL ACID VOLCANICS 119 



played in ledges north of the railroad, on Blue Hill avenue south of 

 Brook street, Milton; near Harvard street and Mount Hope cemetery; 

 in a quarry near Mount Calvary cemetery, on Rutledge road, Rugby, at 

 the intersection of River street and the Providence railroad, and south 

 of Norfolk street and the New England railroad. This tuffaceous material 

 varies from a fine grained consolidated ash to a breccia composed of 

 fragments one to two inches in diameter. In two cases the " aschen 

 structur " which has been described by Miigge* is a feature of the tuffa- 

 ceous volcanic. 



The forms which make up that structure only appear in ordinary 

 light, and are entirely obscured with crossed nicols by the extremely fine 

 quartz-feldspar mosaic which replaces the original fragmental and glassy 

 character of the lava. South of Norfolk street and the New England 

 railroad a light green volcanic, breaking readily into slabs, shows this 

 structure. There is a uniform alteration of the replacing ground mass 

 to pinite, which gives the light pea-green color to the rock. The other 

 occurrence of this structure is in the case of a large boulder of a blood- 

 red color which was found in the woods near Blue Hill avenue, Milton. 



The structure is emphasized in ordinary light by the pigment hema- 

 tite, and obscured in polarized light by homogeneous crystallization. 

 The original fragmental character is, however, indubitable. 



In other instances where the fragmental character of the rock is ob- 

 scured in the hand specimen its obliteration is aided by the uniform 

 alteration of the secondary quartz-feldspar crystallization to pinite. 



With these exceptions, the tuffaceous and brecciated character is always 

 apparent in the hand specimen. The angular fragments exhibit a variety 

 of shades — pink, red, purple, green, and other tints. The fragments are 

 quartz, feldspar (orthoclase, albite), aporhyolite, and spherulites. The 

 fragments of aporhyolite often exhibit remarkably well preserved perlitic 

 parting or fluxion structure or u aschen structur." In the tuff on Blue 

 Hill avenue south of Brook street the fragments are mainly aporhyolitic, 

 and have all been recrystallized and largely altered to pinite and epidote. 

 Orthoclase remains as an original constituent. 



The fragmental character is often obscured, and sometimes completely 

 lost in polarized light, by recrystallization. This is true of all the tuffa- 

 ceous material from the neighborhood of Mount Hope and of Mount 

 Calvary cemetery. In the latter locality the true character of the rock 

 is only obscured, but not destroyed in polarized light, because the crys- 

 tallization of the fragments and the matrix is not uniform. Some frag- 

 ments have a secondary spherulitic crystallization, in which case the 



* 0. Mi'igge : " Untersuchungen iiber die Lenniporphyre in Westfalen und der angrenzenden 

 Gebieten." Neues Jahrbuch f. Min. Geol. u. Pal., B. B. viii, 1893. 



