360 Geological Survey of the Island of Jersey. 



crosses the mouth at b, and comes sharply to the foot of 

 the latter, without the slightest appearance of intermixture. 

 Proceeding south-east from the head of the valley to the point 

 c, where the two main branches of the St. Mary's valley sepa- 

 rate, another spot of junction is obtained, the rocks observing 

 the same order, d is in the St. Laurence valley, at the reser- 

 voir for the mill, one mile south of St. John's. From d, going 

 south and passing e, you arrive at the Town Mills/, in the St. 

 Helier's valley ; at about 60 yards from the mill, the green 

 porphyry overlies the argillaceous schist, in the most decided 

 manner ; this last re-appears at g, on the south side of a slight 

 hollow, close to Mr. Ingouville's, and so continues until it dis- 

 appears under a dark greenstone at h ; on the eastern side of 

 the hill it buries itself finally under the marshes. At Gallow's 

 Hill, just where the St. John's road ascends the brow at i, it is 

 distinctly seen underlaying the green porphyry, as this last 

 stretches out towards Elizabeth Castle. Coasting St. Aubin's 

 Bay, until you arrive at the valley i f , to the south of, and im- 

 mediately underneath the St. Aubin's signal staff, you proceed 

 northward until you reach the elbow j of the St. Aubin's 

 valley, the stream of which crosses the line of separation of 

 the argillaceous schist and sienite, in two or three places ; the 

 last point where I could trace the junction between the two 

 rocks is at k, about 100 yards below the New-road Bridge 

 (Pont Marque). The two rocks can now still be traced to 

 within a short distance of each other until you reach I, the 

 last appearance of the sienite. The sands of the Quenvais 

 render the line from k to I somewhat indistinct. Though the 

 slate underlies the sienite, it frequently pervades the latter in 

 conformable beds, particularly observable on the north-west 

 coast ; at Portelet bay ; at the southern extremity of Fort 

 Regent ; and, above all, at Gouray, where the slate bed is, at 

 least, 100 feet in breadth ; small isolated portions of the schist 

 are also often found completely detached, although in the 

 vicinity. There are fifteen such beds between i and Portelet 

 bay, varying from about 2 feet in breadth to 30 feet, and 

 afford a complete example of the slides and faults in the 

 mining districts ; the beds running about east and west, and 

 the latter north and south. 



