3(38 Geological Society. 



features in it. The author himself puts in the foremost place the 

 fact that this edition contains an Index, which he hopes will be 

 of use to the advanced student, "not merely by enabling him to 

 hud the theorems he is looking for, but also by presentiug the 

 subject in another light." Like theorems are grouped together, 

 and a comparison can be made of the different ways of discussing 

 the same thing. The many additions and improvements which 

 have been made, he writes, will be at once apparent to readers 

 already acquainted with the book. Some of the old references 

 have been corrected and many new ones added. These we notice 

 have in many cases quite confirmed our statement made at the 

 outset. We take our leave of this beautiful book with the hope 

 that Dr. Bouth may yet bring out other editions even more perfect, 

 if it be possible, than this. 



XL VI II. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 133.] 



June 9th {con.). — Dr. Henry Hicks, F.K.S., President, 

 in the Chair. 



3. ' Some Igueous Rocks in North Pembrokeshire.' By J. 

 Parkinson, Esq., F.G.S. 



The acid rocks described in this communication are situated at 

 the east end of the Prescelly Hills. Organisms have occasionally 

 been found, and some discovered | mile west of Crymmych, at 

 the west end of the area, point to an Upper Arenig or Lower 

 Llandeilo age for the deposits. 



The masses of acid rock which occur at Foel Trigarn and Cam 

 Alw show all the characteristics of a true lava-flow. The rock 

 often shows beautifully developed flow-structure. Some of the 

 rocks are very markedly spherulitic and axiolitic. The axiolitic 

 rocks display an interesting structure which the author believes to 

 be best explained by concluding that during the formation of the 

 axiolitic growths a slight movement occurred in the unconsolidated 

 magma. He gives a full account of the variations which mark the 

 spherulitic and axiolitic structures, and also describes nodular bodies 

 produced by flow-brecciation accompanied by very feeble radial 

 growth, and showing irregular but more or less central quartzose 

 areas. 



After describing a brecciated rock, which on the whole is best 

 accounted for by a process of flow-brecciation, due to a second lava 

 breaking up the first while it was still in a plastic condition, the 

 author describes true pyroclastic rocks, and concludes with a 

 description of the conspicuous intrusive diabases, which, in part at 

 all events, seem to be of the nature of a laccolitic intrusion. 



