334 



KEY. E. HILL 0]N T THE ROCKS OE 



The order of geological events in Sark thus appears to have been 

 as follows : — A mass of Archaean rock of uncertain origin had de- 

 posited on it a thick series of beds of alternating materials, princi- 

 pally hornblendic, possibly of volcanic origin. Over these (whether 

 other beds had also been superposed cannot now be discovered) a 

 mass of granitic or syenitic igneous rock subsequently flowed. 

 After the solidification of this, but still probably at a very early 

 period, came a great east and west nip. Except the intrusive dykes 

 there are no later materials with which to continue its history. 



The physical geology of Sark affords many interesting subjects of 

 study. The apparently homogeneous granite, contrary to what 

 might be expected, decomposes much more readily than the banded 

 hornblende-schists. I have been inclined to fancy that on all sides of 

 the island there is a tendency for rock masses, in fault-throws in 

 fissures or in slides, to lean away from the central mass. This 

 may be due to the domed arrangement of the beds, or may have 

 followed the cessation of the nipping force and be a result of elas- 

 ticity, or it may be simply caused by the undermining of the sea. 

 The Coupee is due to a fault and fissure-vein which form a case in 

 point ; and the celebrated Gouliot caves appear to be due to a slide 

 of this nature. ^lore generally, however, the caves have originated 

 in dykes, as have the Boutiques, and, I think, the Creux Terrible. 



The Archaean age of these rocks is not likely to be contested by 

 many. They are analogous in structure to admittedly pre-CambrLm 

 rocks elsewhere. They seem distinctly older than the unfossili- 

 ferous argillites of Jerse}', themselves of extreme antiquity, and, I 

 believe, unlike any neighbouring even Cambrian beds. And the 

 series of the Finisterre beds, which I had the opportunity of seeing 

 this year under the guidance of Dr. Barrois, contains in the whole 

 succession, from Carboniferous down to Cambrian, nothing that can 

 be compared to them. The only rocks I saw there capable of being 

 classed along with the Sark schists were some beds at Pouldu south of 

 Quimperle, which in the French Geological map are themselves 

 marked as pre-Cambrian. 



It may, however, be suggested that their highly crystalline con- 

 dition is due to the influence of the vast mass of granite by which 

 they have been overflowed. I am not sure but that this cause may- 

 have had some effect. However, in the first place the beds nearest 

 the granite are not everywhere the most highly crystalline. 

 Secondly, in the neighbouring region of Brittany I had in 1886 

 the opportunity of studying, under Dr. Barrois's guidance, the effects j 

 produced by intrusive granites on a number of widely differing 

 rocks of almost every age, and none of these were metamorphosed . 

 into any resemblance to these hornblende-schists. Thirdly, the j 

 only rocks of that region which resembled the Herm-Sark-Alderney 

 granite that overlies these schists are granites which Dr. Barrois, on 

 independent evidence, considers Cambrian at latest. 



To myself the most interesting feature in Sark is the unconformable i 

 overlie of the hornblendic beds on older gneiss. Several observers 

 have lately brought forward convincing proofs that certain gneisses j 



