Alfred Harker — VarioUtic Rocks on Carrock Fell. 551 



Plutonic. — Dykes of granite and diorite are found all over the 

 colony. The surfaces of these rocks are often split up and weathered 

 into rounded masses, having a water- worn appearance, owing to the 

 angles and edges being jfirst disintegrated, and then exfoliated, 

 which eventually causes the mass to assume a rounded shape. 

 Amygdaloids are met with in great variety' in the north-west, where 

 they form huge rugged hill masses, upon which nothing will grow. 

 Both the matrix and the enclosures vary much in different places, 

 the latter being most commonly agates and calcite. 



IV. — On some Variolitic Eocks on Carrook Fell. 

 By Alfred Harker, F.G.S., Fellow of St. John's College, Cambridge. 



THE district of Carrock Fell, in Cumberland, is one rich in 

 varieties of intrusive igneous rocks. Those on Carrock Fell 

 itself, while probably belonging to one period of igneous activity, 

 fall under three or four heads, which can be arranged in chronological 

 sequence, as follows : — 



(i.) A large intrusion of gabbro generally laccolitic in character. 



(ii.) A granophyre, also forming laccolitic bodies, but with asso- 

 ciated dykes and veins which traverse the gabbro. 



(iii.) A diabase or gabbro, not essentially different from the first 

 intrusion, but injected later and under somewhat changed conditions. 



(iv.) A large number of dykes and veins of intermediate to basic 

 composition. 



These last are found traversing all the other rocks, and therefore, 

 if we are justified in grouping them all together, they represent the 

 latest intrusions in the neighbourhood. They vary from straight 

 dykes two or three feet wide to narrow and rather tortuous veins 

 not more than an inch in width. 



I have recently presented to the Geological Society an account 

 of the larger intrusive masses of Carrock Fell,^ and I originally 

 intended to include also the minor dykes and veins alluded to. This 

 purpose I abandoned in view of the multiplicity and variety of 

 rocks which they include, but some of the specimens examined seem 

 worthy of a brief notice. 



The only account yet published of any of these minor intrusions 

 is a description by Mr. T. T. Groom, ^ of a one-inch vein traversing 

 the gabbro near the head of Furthergill Sike. This is a tachylite 

 with some remarkable spberulitic structures, and is of distinctly 

 basic composition, having 53-63 per cent, of silica and 20-00 of iron- 

 oxides,^ with a specific gravity of 2*99 (central zone 2-95). I have 

 examined my friend's specimens, but have nothing to add to his 

 description. Veins having a very similar appearance are numerous 



1 Part I., dealing with the gabbro, is published in Q.J.G.S. vol. 1. (1894), 

 pp. 311-336. 



2 Q.J.G.S., vol. slv. (1889), pp. 29S-304. 



^ Analysis by Mr. R. H. Adie (Groom, I.e.). The iron is estimated as ferric 

 oxide, but is probably present in the ferrous state, which would explain the high 

 total of 103-76 per cent. 



