Geol.— Vol. I.] SMITH—SANTA CATALINA ISLAND. 49 



The residues were then filtered off, and the silica was precip- 

 itated in the filtrate by acidifying with hydrochloric acid, and 

 evaporating to dryness. The weight of the silica obtained 

 by this process was compared with the weight of the resi- 

 dues, and except for the shale they all tallied very closely. 

 All lost some silica, and on the addition of ammonia after 

 the precipitation of the silica a slight amount of alumina 

 was precipitated in all the solutions, showing that the alumina 

 in the rock was acted on to some extent. The same test 

 applied to the potassium hydrate (which occasionally con- 

 tains alumina) gave no precipitate. The results showed 

 that the pumice had lost 3.2 per cent, of silica, the volcanic 

 ash 4.2 per cent., while the shale had lost 70.3 per cent. 

 The powdered residue from this shale was subjected to 

 microscopic examination, and with the higher powers was 

 found to contain a large percentage of minute crystalline 

 fragments. Nothing could be made of the isotropic material 

 of the residue. A considerable amount of the residue 

 thrown into dilute acid produces momentary effervescence, 

 showing that a part of it is calcareous, doubtless organic 

 remains. 



Origin of the Shale. — These results show that the shale is 

 largely composed of opaline silica, and, together with the 

 statement of Dr. Hinde, are sufficient to disprove, for this 

 region at least, the hypothesis tentatively advanced by Prof. 

 Lawson^ that the Miocene shale of the coast of California is 

 largely of volcanic origin. That this shale is a part of the 

 same Miocene shale which is found so extensively developed 

 along the coast, there can be httle doubt, although the proof 

 obtained is not positive. It has a similar appearance, presents 

 the same variations, contains abundant micro-organisms (a 

 characteristic feature of the Miocene shales), while the 

 occurrence of fish scales adds another link to the chain of 

 evidence, as this is another marked characteristic of the 

 coastal shales. Further, the Miocene shale occurs at San 



i"The Geology of Carmelo Bay," by Andrew C. Lawson. Bull. Dept. Geol., Univ. 



Cal., Vol. I, pp. 24-26. 



(4) January II, 1897. 



