250 



J. F. XEWSOM — CLASTIC DIKES 





-,^^S^^^^^^ 



:,i^-^. 



been observed along the coast in this region. This intrusion was ob- 

 served by Dr Ralph Arnold, and the following notes and diagram 

 (figure 18) are supplied by him. 



The sea-cliff is 100 feet high where cut by the dike. The upper 40 feet 

 of the cliff is made up of Pleistocene sands, gravels, and clays, resting 

 unconformably on the diatomaceous Miocene shales. The intrusion has 

 an even thickness of 1 foot from the bottom to the top of the diatoma- 

 ceous shales. The sides of the fissure inclosing the dike are perfectly 



straight and clean cut. There is no 

 displacement in the shales on the op- 

 posite sides of the intrusion. 



The dike is composed of rather fine, 

 reddish, compact sand showing a trans- 

 verse, slightly wavy, banded structure. 

 The material of the dike is totally un- 

 like that of the overlying Pleistocene 

 beds and could not have been derived 

 from them. 



Two small dikes, 53, 54, are exposed 

 at the east side of the public road 75 

 yards south of Coast post-office, and 

 two others, 55, 56, are exposed at the 

 roadside just east of the crossing at 

 Majors creek. 



The number of intrusions exposed 

 along the"coastline west of Santa Cruz and at the asphalt rock quarries 

 northwest of there indicate that the diatomaceous shales of that region 

 are cut by great numbers of dikes whose outcrops are obscured by 

 the soil. 



S UMMA RY AND CONCL VSIONS 



Figure 18. — Diagram of Dike 57. 



Exposed near the mouth of Scott creek, 

 Vi miles northwest of Santa Cruz. This is 

 the westernmost dike observed in the sea- 

 cliff northwest of Santa Cruz. 



1. The dikes of the Santa Cruz region cut the diatom shales at angles 

 varying from to 90 degrees. The shales are much jointed in this region. 



2. In composition the dikes vary from fine highly bituminized sands 

 to soft sandstones with no bituminous odor. They often contain inclu- 

 sions of diatom shale. 



3. In size they vary from a mass 600 feet across to mere films along 

 joint planes. The small films are usually more or less highly bitumi- 

 nized, while the larger intrusions are usually comparatively free from 

 bitumen. 



4. There is usually an irregularly banded and sometimes a columnar 

 structure in the larger intrusions which are practically free from 



