OK Tin: SOUTH OF TIIK ISLK OF MAN. 433 



Silverburn on tlio north-west, thence, bounded on tlio west by a 

 fault whicli throws them down against the slates, to Strandliall. 

 Hounded on the soutli by the sea, they extend eastwards from Strand- 

 liall to the i)eniiisula of Langness, where they are thrown down 

 against the slates by two faults. From Langness northward they 

 are bounded by the sea. They consist, in descending order, of : — 



4. Poolvash black marble beds (with interbedded volcanic tuii') = 

 i^osidonian schist of Cumming. 



3. I' pper or Poolvash limestone. 



2. Lower or Castletown limestone. 



1. Conglomerate and sandstone. 



Numbers 2 to 4 belong to the Carboniferous Limestone series. 

 No. 1 was held by Cumming to be of Old lied Sandstone age, but 

 is more probably of Calciferous Sandstone age, as shown by Horno *. 



III. Types of Igneous IIocks met with. 



Omitting from consideration the granite which occurs at Foxdale, 

 as well as north of Laxey, we have evidence of at least five and 

 possibly six distinct series of volcanic rocks differing both in com- 

 position and in age. The earliest series is : 



(a) I'he Diabase Series. — This is best seen in the peninsula of 

 Langness, and is termed by Cumming " greenstone." It is certainly 

 of pre-Carboniferous age, as it never penetrates the basal Carbon- 

 iferous conglomerate. It forms dykes and intrusive masses in the 

 Silurian slates and often follows closely their line of strike, so that 

 its intrusive character is not obvious at the first glance. The 

 general direction of strike is north-easterly, being thus quite dif- 

 ferent from that of the olivine-dolerite dykes, which is north- 

 westerly. I have not worked out the details of the diabase dykes, 

 <S:c. 



(b) The Micro-granite Dijlce.—On the hillside S. 41° W.f of Crosby 

 lailway -station a micro-granite dyke 24 feet wide cuts through the 

 Silurian slates with a northerly strike, and is exposed in two small 

 quarries worked for road-metal. The slates have been altered at 

 the point of contact into a micaceous schist. The interesting feature 

 of this dyke is the parallel structure of its salbands. According to 

 Prof. Boyd Dawkins's manuscript notes and map, the general 

 direction of strike of this dyke is north-easterly, and it can be 

 traced south-westward by St. llunn's Church and St. Patrick's 

 Chair to Windy Common. 



(c) The Aug ite-porphy rite Series. — This is exposed in a narrow 

 strip a mile and a quarter long, extending from Scarlet Point, south- 

 west of Castletown, in a north-westerly direction to Poyll Vaaish 

 (Poolvash). It consists of tuff, breccia, agglomerate, bedded lava, 

 and intrusive masses, the whole forming " a small ancient volcano, 

 dissected and laid bare," to quote the Eev. J". C. AVard J. Some of 



* Trans. Edin. Geol. See. toL ii. (1874) p. 327. 



t All coiupass bearings are corrected 21° west for magnetic declination. 



X Geol. Mag. (1880) p. 5. 



