428 J. S. DILLEB AM'-MNi: DIKES. 



of distinct grains, but also aa finer material mingled with the cement in the 

 interstices of the larger grains in such a way as to suggest at times that it 

 is a part of the cement and deposited since the formation <>f the dikes. As 

 there are no quartz veins found about the dik* - pting the extremely 



minute ones which traverse in each case only a single grain of sand such as 

 is derived from metamorphic rocks, it is believed that the dike rocks have 

 not received dep - - if silica from solution in circulating waters. The 

 tenting Bubstance of the rock is carbonate of lime, which is abundant in 

 the adjacent shale- and forms larger or smaller parts of all the sandstone 

 dikes, occasionally occurring as small veins. Grains of eruptive rocks are 

 very rarely observed. In section 2378 i- a fragment of hornblende andesite. 



Some associated Cretaceous Sandstone Beds. 



Some of the fine-grained sandstones clearly interstratified with the shales 

 of the Horsetown and Chico beds contain Bcales of mica, and in nearly every 

 respect excepting mode of occurrence so closely resemble the dike rocks that 

 hand specimeus of the two cannot be readily distinguished without the aid 

 of a microscope, and even then it is often impossible. Such saudstoni 

 are, however, not common. They have been observed on Byron creek at 

 the top of the cascade, half a mile above Ono. Their strike is X. 10° E., 

 dip 20 S. E. Specimen- 2548, parallel to the bedding, and 2549, perpen- 

 dicular to it, were collected here. They occur also two miles north of' Ono, 

 on the road to [go, where the rock i- very micaceous and rests directly upon 

 the dioritic rock from which it has been derived ; strike X. 40° E., dip 22 

 S. E. Specimen 1991 wa- collected at this locality. 



Similar rocks were observed at the .lam on .Middle fork, where specimens 

 2532, parallel, and 2533, perpendicular to the bedding, were collected. The 

 >trike b ff.24 W.. and dip 32 X. E. <)u Dry creek, 3 miles above A. 

 Allen'-, specimen 251 1 was found. Such rock- occur also on Salt creek, 

 half a mile above Martin'.-, with a strike of N. 30 to 37 W., and dip 2 

 to 30 N. E. Specimen 2517 was collected at this locality. The last local 

 itv t'» !"■ mentioned is on Middle fork, a mile above its mouth, where speci- 

 men 2537, which is quite full of mica, wa- found. 



The locality last named i- to the eastward of any of the dikes, ami 



atigraphically above them. The beds on Salt creek and at the dam on 

 Middle fork are penetrate. 1 by the dikes apparently without change, but 

 those of the other localities which lie northwest of the .lik.- area dip easterly 

 toward- the dikes and may possibly reach them at considerable depths be- 

 neath th( surface. 



The mineralogical composition and structure of all th< -.■ sandstone bed- is 



essentially the same. In composition, al-o, they n semblc the sandstone dikes, 



but in minute structure they difier in an important respect: tn the sandstone 



