264 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the magma developed along relatively deep slate contacts of the batho- 

 lith. If we may include the dark colored, porphyritic knots which 

 are, as will be shown, very closely related chemically and mineral- 

 ogically and which are believed to be of identical origin with the 

 rhombenporphyry we may say that, a relatively basic feldspar- 

 porphyry phase was developed not only against the deeper projections 

 of slate but also to a small extent against the deeper contacts of the 

 aporhyolite, and in locations such as those just underneath the granite- 

 porphyry in the higher levels of the contact zone, where the magma 

 remained fluid for a sufficient length of time under the cover of its 

 own porphyritic phases to permit differentiation to take place. 



Distribution. — The areal distribution of this rock is relatively very 

 small. It is limited in its occurrence to the Pine Hill and the Pine 

 Tree Brook areas, in both of which it occurs at or near the slate con- 

 tacts, and is associated with developments of the fine-granite and with 

 abundant xenoliths in the adjoining granite-porphyry and porphy- 

 ritic granite. None of its outcrops are continuous over any consider- 

 able area and it is doubtful if any single mass of it has a continuous 

 area of over 10,000 square feet, or a thickness measured by more than a 

 few tens of feet. The majority of its exposures are considerably 

 smaller than the figure just mentioned and the total volume of the 

 rock is relatively small. The best exposure for study and collection 

 of material is found in the northern part of the Pine Hill area (see 

 special map), at the end of a short street which runs east from Willard 

 Street in West Quincy. At this point is a prominent ledge which in 

 the past has been worked for road metal. The rock has been quar- 

 ried in an open cut which gives excellent exposures for a distance of 

 some 50 feet in length and to a depth of about 15 ft. Going east 

 from this ledge toward the railroad track several other smaller 

 masses of it occur. These are situated very close to two small out- 

 crops of Cambrian slate and are perhaps in contact with it. They 

 are, as shown by abundant contacts, enclosed in a granite-porphyry. 

 At a point a few hundred yards southwest of the first mentioned 

 outcrop a number of smaller masses occur enclosed in the porphyritic 

 granite of the contact type. The largest of these masses will not 

 measure more than a few feet in their greatest dimension and from this 

 they grade down to fragments 'which are comparable in size to the 

 larger xenoliths found in the granite generally throughout this area, 

 as will be noted later. Small dark patches are abundant in the 

 granite of this type from this location along an east-west line as far 

 as the railroad where, in a large and finely glaciated ledge, the inclusions 



