458 MR. A. HARKEE ON THE ERUPTIVE ROCKS IN THE 



"We pass at once from perfectly typical homblende-picrite to a rock 

 in which no olivine can be detected, while felspar, instead of being 

 locally and rather sparingly present, becomes essential and abundant. 

 Nevertheless it cannot be doubted that the two rocks form parts of 

 one and the same intrusive mass. Mr. Tawney, who noticed that 

 the hornblende-diabase (" diabase " in his description) occurs both 

 below and above the hornblende-picrite (his " olivine-diabase "), 

 suggested doubtfully that the olivine-bearing rock was intruded into 

 the other ; but his examination of the locality was rather a cursory 

 one, and he appears not to have distinguished between the hornblende- 

 diabase and the felspathic variety of the picrite. The rocks have, 

 indeed, many characters in common; but what unites most clearly 

 the picrite and the hornblende-diabase as the products of one igneous 

 intrusion or set of intrusions is the behaviour of the segregation- 

 veins at the junction. These veins, already mentioned, pass from 

 one rock to the other, so that it is impossible, where they occur, to 

 draw any line of demarcation. 



The direction of the plane of division of the hornblende-diabase 

 and hornblende-picrite is also the direction of the banks or quasi- 

 strata of the latter rock, marked by the alternation of different 

 lithological types ; also of the thin olivine- veins, and of the coarse 

 segregation-veins when these show any regularity of disposition. 

 Further, the base of the whole igneous mass is a plane parallel to 

 the above and agreeing with the stratification of the sedimentary 

 rocks below, so that the actual junction is a definite bedding-plane 

 of the shales (fig. 4). The mode of occurrence of the mass is there- 

 Pig. 4. — Ideal section through the southern end of Mynydd 

 PenarfynydA. (Length | mile.) 



N.N.W. B.S.E. 



A. Upper Arenig strata. 

 D. Diabase of Tyn-y-rhedyn sheet. 



HD, Hornblende-diabase, and P, Hornblende-picrite of the Penarfj-nydd 

 laecolite. 



fore similar to that of the well-known " laccolites " of Southern 

 Utah *, with the difference that, instead of being homogeneous, it 

 ■^ " Geology of the Henry Mountains," U. S. Geol. Surv. 1880. 



