238 



J. F. XEWSOM — CLASTIC DIKE? 



bitumen exudes from the sands. The rock, which is brittle to a sharp 



blow, is plastic under a gentle or even pressure. Onl}^ a few of the more 



interesting of the many intrusions at 

 the asphalt quarries are described 

 and figured below. The locations of 

 the quarries and of the dikes that 

 are figured from these are shown in 

 figure 6, and at A, figure 5. 



In thickness the dikes at the quar- 

 ries vary from mere films along joint 

 planes to intruded masses fully 10 

 feet thick. Some of the thinnest of 

 the intrusions composed of bitumin- 

 ized sands follow joint planes for 30 

 or 40 feet from the parent sandstone 

 bed below. It is remarkable to what 

 distances and into what thin cracks 

 these sands have in some cases been 

 forced. 



In some instances the dikes branch 

 and the branches again coalesce (see 

 figures 7, 8, and 9). In this respect 

 they resemble dikes of igneous rock 

 and the}^ have doubtless been formed 



in much the same way, except that intense heat has not been necessary 



to give plasticity to 



the injected material. 

 At the east side of 



the main or Point 



quarry the bitumin- 

 ous sandstone and 



the overlying shales 



are faulted, the down 



throw of about ten 



feet being on the east. 



Along the fractured 



zone and above the 



sandstone there are 



large masses of in- 



truded sandstone 



with shale inclu- 

 sions. 



FiGrKE 6. — Diagram of the Asphalt Quarries. 



These quarries of the City Street Improve- 

 ment Company are five and one-half miles 

 northwest of Santa Cruz. The heavy black 

 lines show the quarries. The numbers refer 

 to the dikes that are described and figured 

 below. 



< — ^w 



=< =^ 



f 



E DIATOM SHALESJ 



^^ 



=J- r= 



~^^^ — ^ 



^^^^— DIATOM SHALES — 



LH — — ^ ^ ^f ■ 1 



-^ ^-"sT^s^ ^^^^z^^^BB 



Figure 1.— Bituminous Sandstone Dike at Point Quarri/. 



This picture was taken in August, 1901. The top of the main 

 bituminous sandstone bed from which the dike is derived is shown 

 in black at the bottom of the picture. 



