98 



University of California. 



IVOL. I. 



From point A northward as far as D, the diabase presents puz- 

 zling variations, which it is difficult to account for, save on the sup- 

 position that the rock at the time of its eruption did not move as a 

 mobile molten liquid, but as a somewhat viscous mass, composed of 

 a magma heavily charged in places with solid, or partially solid, 

 fragments, of a compact spheroidal rock resembling the basalt 

 below. Upon cooling, the magma crystallized in its deeper-seated 

 portions as the normal diabase, filling all the interstices between the 

 fragments, and forming, what are now the more massive portions of 

 the cliff Whether the included fragments were formed from the 

 magma itself by a partial cooling followed by renewed movement, 

 or whether they were derived from the spheroidal basalt below, it 

 does not seem possible to determine. From certain internal evi- 

 dence afforded by one of the spheroids the former hypothesis seems, 

 however, most probable, and Miss Raisin * has advanced a like sug- 

 gestion to account for a similar association. Certain it is, that in 

 passing along the foot of the cliff, spheroids composed of aphanitic 

 rock are encountered at various places, so intimately associated 

 with the coarsely crystalline diabase that it is impossible to sepa- 

 rate them into two distinct portions. 



Aside from these apparent inclusions, the diabase presents its 

 usual macroscopic appearance of a dark green distinctly crystalline 

 rock. Slides from between C and D exhibit the distinctive char- 

 acter of the diabase, including the presence of iddingsite. The 

 augite is more deeply colored here than elsewhere, and more 

 strongly pleochroic. At point E the diabase becomes more finely- 

 crystalline and shows abundant white ainygdules. Under the 

 microscope, it is seen to be composed of lath-shaped plagioclases, 

 with the augite in scattered irregular grains, and is evidently a 

 transition rock, indicating an approach to surface conditions of 

 cooling. It is much decomposed and full of calcite in the form of 

 amygdules, veins, and replacements. This conspicuous calcification 

 is a marked macroscopic feature of the diabase from E northward, 

 showing itself in veins and stringers throughout the shattered mass. 



A specimen from the central portion of one of the spheroids 



*The Variolite of the Lleyn and Associated Volcanic Rocks, Q. J. G. S-, 

 Vol. XLIV, p. 152. 



