Wadsworth.] 234 [May 16, 



SOMERVILLE, QUARRY NEAR THE CAMBRIDGE ALMS HOUSE. 



Composed of viridite, feldspar, magnetite, and some augite. 



SOMERVILLE, WEST CORNER OP WALNUT AND PEARL STS. 



This is composed principally of viridite and magnetite, with traces 

 of feldspar, augite, and apatite. 



SOMERVILLE, FREMONT ST. 



This rock is finely porphyritic from elongated feldspar crystals, the 

 matrix being composed of clinoclase, viridite, and magnetite. 



ARGILLITE QUARRY, ELM ST., SOMERVILLE. 



Composed of viridite, magnetite and feldspar, forming a matrix in 

 which large feldspar crystals are embedded; these crystals are much 

 altered, and are changed to viridite in part. 



SOMERVILLE, WIGGLESWORTH ST. 



Surface specimens show the "greenstone" to be composed mainly 

 of viridite and magnetite, with a little quartz and calcite. 



EAST CORNER OF MARSHALL AND PEARL STS., SOMERVILLE. 



Much decomposed, composed almost entirely of viridite and mag- 

 netite. 



"Porphyry." 



SOMERVILLE, FREMONT ST. 



The dike here is several feet in width, very coarsely porphyritic on 

 the underside, becoming finely so towards the upper parts of the 

 dike, where it is a compact gray mass. The porphyritic structure is 

 owing to yellow and grayish white pseudomorphs after feldspar, au- 

 gite and hornblende crystals; the pseudomorphs being composed 

 principally of a hydrous, ferruginous, and argillaceous material con- 

 taining calcite, and generally showing under the microscope a some- 

 what fibrous structure. The base is a mottled grayish mass, now 

 composed principally of calcite and argillaceous matter. 



ARGILLITE QUARRY, ELM ST., SOMERVILLE. 



The dike here is narrow, and only finely porphyritic. Thin sec- 

 tions reveal a similar structure to the " porphyry" on Fremont Street, 

 except that the alteration has not extended quite as far. A close 

 resemblance exists in this and the upper part of the one on Fremont 



