1904.] HAEHL AND ARNOLD — THE MIOCENE DIABASE. 23 



section throughout nearly the whole diabase area. At its base is 

 the conglomerate, consisting of water-worn diabase pebbles ce- 

 mented by a more or less siliceous sand. In some places, however, 

 the amount of diabase is so great that it is difficult to distinguish 

 the beds, where badly weathered, from the diabase tuff. 



The presence of barnacles {Bahifius) and of a single specimen 

 of Pecten afford the best evidence of the sedimentary nature of 

 the deposit, and fix its origin as marine. The conglomerate is 

 not always of this nature, however. South of the Alpine school- 

 house the base of the Purisima consists of a rather incoherent 

 shale breccia, which had its origin in a talus slope. Part of a 

 Balanus was found in this breccia, showing that at least part 

 of the deposit was laid down under water. In places fragments 

 of the Miocene shale, together with hardened sandstone and 

 chert, make up the greater portion of the basal layers, indicat- 

 ing possibly that the Purisima coast line lapped over the intruded 

 area and obtained its materials, not from the diabase area, but 

 from beds to the east of them. The total thickness of the Puris- 

 ima formation is probably about seven hundred feet. As a rule 

 the conglomerate beds are thin ; the thickest of them are about 

 twenty feet in thickness. At some localities, notably at a place 

 a quarter of a mile southwest of the Alpine schoolhouse, the base 

 of the Purisima consists of shale, which rests unconformably upon 

 the diabase. 



Above the conglomerate is a thin bed (four or five feet) of soft 

 green sandstone, stained by the chloritic weathering products of 

 the diabase. It contains bones and sharks' teeth and, in some 

 localities, a rich marine invertebrate fauna. Over the green sand- 

 stone is a bed of an unfossiliferous, nodular shale of perhaps two 

 hundred feet in thickness. 



On the top of the unfossiliferous shale are sandy shales and fine 

 sandstones probably five or six hundred feet thick. While these 

 may readily be distinguished by their lithology, they are also char- 

 acterized by numerous fossils which are in a fair state of preserva- 

 tion. The following species were gathered from the Purisima beds 

 in different parts of the area under discussion : 



List of Fossils from the Pu?'isima {Pliocene) Formation. — Those 

 marked with a (*) are characteristic, so far as known. — 



