766 KEPORT— 1893. 



Great interest attaches to the remarkable change in character and composition 

 ■which the rock undergoes on passing from its eastern to its western boundary : 

 quartz and orthoclase as well as plagioclase felspar appear as additional consti- 

 tuents ; simultaneously the hornblende becomes actinolitic, and gives rise to a 

 profusion of black mica. From an amphibolite the rock changes into a quartz- 

 mica-diorite. 



Numerous veins of quartz traverse the adjacent schists, and can be traced on 

 the western side of the amphibolite up to and into it ; they contain potash felspar 

 near the junction, and it is to their influence that the transformation of the 

 amphibolite is unquestionably due. 



Another instance is thus afforded of a rock of intermediate type resulting from 

 the admixture of already differentiated material. 



6. On some Igneous Hoclcs of South Pemhrohesldre, luith a Note on the Hocks 

 of the Isle of Qrassholme. By F. T. Howard, B.A., and E. W. Small, 

 M.A., B.Sc. 



I. Constitution of the District. — The district is largely composed of rocks of 

 Old Red Sandstone age with smaller patches of Silurian and Ordovician strata, 

 bounded on the N. by Carboniferous rocks. The igneous rocks may be roughly 

 divided into two groups : the first or northern group runs in a more or less E. 

 and W. direction, and marks the southern boundary of the Culm, while the second 

 (in the S.W.) occurs as isolated patches, associated principally with rocks of 

 Silurian age. 



II. Reference is given to the previous work of Kidd, De la Beche, Murchison, 

 Aveline, Rutley, Hicks, Davies, and Teall. 



III. Detailed Descriptio7i of the Igneous Hocks. — (A) Northern group. — Sub- 

 divided into three distinct patches: (a) In S.E. running from Benton Castle 

 N. W. to Rosemarket, and from Benton Wood to Waterless. Practically all the 

 rocks of this patch are quartz felsites or rhyolites ; several of them show good 

 flow structure and spherulites ; much alteration has occurred in places, and the 

 rocks have become brecciated. At Waterless is a rock (marked as granite on the 

 Survey map), composed of quartz and felspar, whose connection with the felsites 

 is not very clear. (6) To N. .and E. of the last, stretching from near Llangwm 

 to beyond Tier's Cross. The rocks of this patch show great alteration, readily 

 weathering down to a felspathic gravel. On the Survey map the patch is 

 marked partly as syenite and partly as greenstone. Dr. Hicks describes the de- 

 composed form of the rock seen in the railway cuttings at Johnstone as a 

 granitoid rock, very similar to the Dimetian. Two large quarries at Annikell 

 atod Targate give the only good exposures of the unweathered rock, chiefly a coarsely 

 crystalline aggregate of quartz and hornblende, with a gneissose structure. 

 Microscopically some portions might be described as typical hornblende schist, 

 others as quartz diorite, granitic in aspect. Mr. Watts suggests that this rock 

 may possibly be allied to the soda granites of Leiuster, described by Professor 

 Sollas. Another rock, a highly quartzose granite with microcline, appears to be 

 later than the diorite : it shows much evidence of crushing and straining. A 

 greenish black fine-grained rock (macroscopically appearing to be an ordinary 

 dolerite, microscopically showing fresh stumpy plagioclase set in large plates of 

 hornblende, apparently of primary origin) seems to be intrusive into the quartz 

 diorite. It is doubtless the rock referred to by Dr. Hicks as diabase ; it should 

 perhaps be called an epidiorite, or proterobase. {c) The third portion of group 

 A runs from Romans Castle on a narrow strip past AValwyns Castle up to 

 Talbenny, where the outcrop broadens and forms the cliffs of Goultrop Head. 

 As described by Dr. Hicks, Mr. Davies, and Mr. Teall, the main rock appears to 

 be an altered quartz diorite penetrated by a whiter granite ; besides which there 

 seem to be basic dykes, all more or less altered, some epidiorite, others hornblende 

 schists. The rock at Walwyns Castle appears, however, to be afelsife. 



(B) Southern group. — This group consists of a number of patches, which 



