WILSON: MEDFORD DIKE AREA. 
The next intrusion of the region was that of the Medford dike (de¬ 
scribed by Hobbs and others), a hornblende-biotite-angite diabase, very 
coarse and much disintegrated. The lenticular quartz inclusions have 
already been described by Dr. Jaggar (’98, p. !207). There occurs 
on the west flank of Pine hill a large inclusion of a coarse granular 
quartzite, readily friable, and almost pure white, the individual 
'grains of quartz being about one sixteenth of an inch in diameter. 
There is also a large fragment (approximately two hundred and 
sixteen cubic feet) of sandstone exposed in a small excavation just 
south of the Big Quarry. This still retains obscure traces of bed¬ 
ding planes, and, in addition to the large amount of clastic quartz, 
carries a few rounded grains of pale green augite and a very small 
amount of secondary p^u-ite. At Pasture hill, Medford, in about 
the middle of the dike, some laro’e inclusions have recentlv been 
disclosed by quarrying. They are very much jointed so that it is 
difficult to obtain a freshly fractured surface, but they are probably 
fragments of one of the earlier diabase dikes, one of which, about 
one hundred and fifty feet in width, is cut through at this place. 
Tn the area studied, there is a single dike of fine-grained diabase 
cutting both the Medford diabase and the adjacent granite. The 
chief components are augite, plagioclase, magnetite, and secondary 
chlorite. 
In the Middlesex Fells, along a line at right angles to the axis of 
the dike, the horizontal displacement accompanying the intrusion of 
the Medford diabase varies to a maximum of about two hundred 
and fifty feet. The older dikes on the east side are all offset towards 
the north, the maximum horizontal offset parallel to the axis of the 
Medford diabase dike being about one hundred and seventv-five feet. 
As the simple gaping of the crooked fracture occupied by the big 
dike would not offset the oldei* dikes more than twenty-five feet, it 
is probable that there has been movement in more than one direction. 
Just what the amount and direction of throw is, cannot at present 
be stated. Possiblj" this displacement is one of the causes of the 
production of the joint planes in the older diabase dikes, although 
the Medford dike is itself jointed, in part by contraction while cool¬ 
ing and solidifying. - 
Summary. — (1) There are series of stratified rocks in this locality 
older than all the volcanics. In addition to evidence alreadv cited, 
proof of older sedimentary rocks is afforded by the occurrence of 
