Dodge.] 416 [February 3, 



The felspar is white, flesh-colored, red or gray; often well crystal- 

 lized. One specimen has crystals in which one face shows a white 

 layer between two parallel red layers. This ledge and similar ones 

 in Medford, and under the old powder-house on Winter Hill in Som- 

 erville, decompose readily, giving the familiar appearance, when the 

 gravel is cleared away, of rounded masses piled upon one another. 

 Where the intrusives cut the slate, pyrites is often developed in the 

 latter near the dykes. 



A kind of diorite very common in Newton, is fine grained, grayish 

 or greenish, and has a specific gravity of 2.77, containing therefore a 

 large proportion of felspar; hornblende being about 3.3 and felspar 

 2.4 or 2.5. This and a cryptocrystalline variety which is grayish in- 

 clining to purple, are frequently amygdaloidal. 



These amygdaloids, though common throughout the vicinity, are 

 particularly abundant in the two large isolated masses of hornblendic 

 rocks, one occupying a district on both sides of the horse car line 

 of Brighton, extending about two miles east and west; the other be- 

 tween Otis, Murray, Homer, Fuller, Maple and Auburn Streets, in 

 Newton. The direction of the long, narrow, and nearly continuous 

 area which they occupy here would seem, at a casual observation, to 

 put them in the same category with the parallel ranges of crystal- 

 lines; but the position of the stratified rocks about them, and their 

 own disposition, distinguish them from the very similar rocks, for 

 instance, two miles south in Newton and Brookline, referred to above 

 on page 375. Whether these rocks as a whole are of the crystal- 

 line stratified class, or whether they have undergone such change 

 as to entitle them more appropriately to a different name, this is cer- 

 tain, that some of them are found cutting the slate and conglomerate, 

 and poured out over them, and that near the contact the slate is 

 greatly hardened. 



There is a kind of concretionary arrangement of curved bands and 

 wavy layers in the mass, sometimes seen, which shows best when the 

 rock has been fractured in a smooth plane. Such " ribboned lavas " 

 may be seen at the south-east corner of Beacon and Harvard Streets 

 in Brighton. 



The nodules of these amygdaloids, which are usually small, rarely 

 as large as a bean, contain, as usual, quartz, calcite, anhydrite aud 

 chlorite. These, with epidote, prehnite, the zeolites, apatite, copper 

 and iron pyrites, are the most common crystalline minerals of the 

 eruptive series. Intrusions of quartz, as well as the quartz veins 



