R ANSOME. 



Eruptive Rocks of Point Bonita. 



87 



in Fig. 3, where it is intrusive into the spheroidal basalt, and, for a 

 few inches only, into the overlying pyroclastic rock. There is also 

 little doubt but what it composes the mass of the cliff above the 

 fragmental layer, but the forbidding character of the rock wall did 

 not invite closer inspection. 



At A the land is low, and the eruptive rock disappears, but near 

 point B the diabase again cOmcs into view, and continues exposed, 

 as a narrow strip, up to the sand-bar of Rodeo Lagoon, presenting 

 at several points clear evidence of its intrusive character as regards 

 the sandstone and jaspers, and also passing into modifications which 

 resemble closely the typical forms of the spheroidal basalt. In the 

 corner just north of C, sandstone and shale are exposed near the top 

 of the cliff in a manner to indicate the intrusion of the eruptive 

 rock, but in the little notch between D and E the relations are much 

 more clearly shown. Sandstone and jaspers are here seen to be rest- 

 ing upon, and partly imbedded in, the eruptive rock. The jaspers 

 have been much disturbed, and their layers crumpled and contorted, 

 presenting, close to the contact, a rounded, semi-fused appearance. 

 The sandstone is much darkened in color, considerably sheared, and 

 the shaley bands curved and twisted. At this particular point jas- 

 pers may be seen overlying the sandstone, but this does not seem to 

 be their true stratigraphical position; the abrupt manner in which 

 the two beds terminate suggests rather, that they are disrupted 

 portions of strata which have been thrust one over the other dur- 

 ing the period of eruptive activity. An inconsiderable amount of 

 typical serpentine was noted, apparently between the eruptive rock 

 and a part of the sandstone. Similar occurrences of this rock were 

 also observed in two other instances where the sandstone is in con- 

 tact with the volcanic rock, but the patches were too small and 

 obscure to suggest their origin or relationship. 



From E northward, the red jaspers may be seen piled in confu- 

 sion along the top of the cliff, with a general easterly inclination, 

 and separated from the eruptive rock below by an irregular intrusive 

 contact. The latter rock frequently includes fragments of the former, 

 and has forced itself, in long, ragged tongues, between the thin sili- 

 cious sheets, breaking and crumpling them, and giving their gener- 

 ally sharp edges a rounded and fused appearance. That there has 



