GEOLOGY OF THE COAST DISTRICT. 633 



on Pawtuckaway, Whitney says, in Jackson's report, — "On the lower 

 mountain there occurs a dyke of greenstone trap, wliich crosses its sum- 

 mit and divides it into two nearly equal parts. This dyke is singularly 

 columnar, and, on the face of a bare ledge, inclined about 45°, it assumes 

 the form of steps, fifteen or sixteen in number, each about nine inches in 

 height. They are known to the inhabitants as the 'Stairs.' It varies 

 from six to twelve inches, and was traced for a quarter of a mile." Final 

 report, page 30. Jackson speaks of other dykes on the sea-coast north 

 of Little Boar's Head, running in a north-east direction, and varying in 

 width from a few inches to ten feet. 



One of the most interesting dykes seen near Portsmouth is one of 

 sienite twenty-five feet wide, near the powder magazine, two or three 

 miles out from the post-office. It has the course N. 50° E., and seems 

 to be conformable with the schist adjacent. The phenomena indicate 

 that this diorite, lithologically the same with much of the Exeter group, 

 was erupted after the deposition of the Merrimack group, probably dur- 

 ing the epoch of its elevation. I observed it for 200 or 300 feet in length. 

 Such facts suggest the origin of the Exeter rock subsequently to the 

 deposition of the Merrimack series. 



There are several trap dykes on Frost's point. Rye. One of them is 

 fifteen feet wide, with many small, irregular branches. Loose specimens 

 near by seem like labradorite diorites. Several have been noted in Sea- 

 brook, as in the village and at the school-house to the south-east. A 

 short distance north of the village there is a beautiful porphyritic trap, 

 where the feldspar crystals are like kernels of rye, of light color, scat- 

 tered through the black base. 



Little has been done in the study of the trappean rocks, as we have 

 been waiting to ascertain their proper mineralogical designations, which 

 will be given under the head of Lithology. Meanwhile we may call at- 

 tention to an interesting case of dykes crossing one another at Bald Head 

 in York. The first eruption was of a large, porphyritic trap dyke, with 

 the course N. 55° E.; the second kind is more compact and fine-grained, 

 running north-east, but irregularly ; the third series cuts across both the 

 others, and must therefore have been injected at a later epoch. The ma- 

 terial is a brown scoriaceous trap. These three kinds of rock occur in 

 our state, and belong to the same epochs of eruption with those in York. 

 VOL. iL 80 



